feat: add linux install script for manual installation support (#137)

### Description

Add linux install script for manual installation support. This can
install from a local zip or from our release downloads.
Examples:
```
$ bash ./scripts/install_linux.sh 
Downloading latest release from GitHub
  % Total    % Received % Xferd  Average Speed   Time    Time     Time  Current
                                 Dload  Upload   Total   Spent    Left  Speed
  0     0    0     0    0     0      0      0 --:--:-- --:--:-- --:--:--     0
  0     0    0     0    0     0      0      0 --:--:-- --:--:-- --:--:--     0
100 34.0M  100 34.0M    0     0  42.6M      0 --:--:-- --:--:-- --:--:-- 42.6M
Creating system install folders. This may ask for your password...
Leaving existing observe-agent.yaml in place.
Observe agent successfully installed to /etc/observe-agent

$ observe-agent version
Using config file: /etc/observe-agent/observe-agent.yaml
observe-agent version: 1.4.0

$ ZIP_DIR=dist/linux_amd64_v1/observe-agent_Linux_x86_64.tar.gz bash ./scripts/install_linux.sh 
Installing from provided zip file: dist/linux_amd64_v1/observe-agent_Linux_x86_64.tar.gz
Creating system install folders. This may ask for your password...
Leaving existing observe-agent.yaml in place.
Observe agent successfully installed to /etc/observe-agent

$ observe-agent version
observe-agent version: 1.4.0-SNAPSHOT-48bafce6
observe-agent config file: /etc/observe-agent/observe-agent.yaml
```

### Checklist
- [ ] Created tests which fail without the change (if possible)
- [ ] Extended the README / documentation, if necessary

Author: obs-gh-mattcotter

scripts/install_linux.sh

@@ -0,0 +1,73 @@
+#!/bin/bash
+
+set -e
+
+agent_binary_path="/usr/bin/observe-agent"
+observeagent_config_dir="/etc/observe-agent"
+tmp_dir="/tmp/observe-agent"
+
+# If the observe-agent.yaml file already exists, leave it alone.
+# Otherwise we need to know what the collection endpoint and token are.
+if [ ! -f "$observeagent_config_dir/observe-agent.yaml" ]; then
+    if [ -z "$OBSERVE_URL" ]; then
+        echo "OBSERVE_URL env var is not set"
+        exit 1
+    fi
+
+    if [ -z "$TOKEN" ]; then
+        echo "TOKEN env var is not set"
+        exit 1
+    fi
+fi
+
+# If the zip file is not provided, download the latest release from GitHub.
+if [ -z "$ZIP_DIR" ]; then
+    echo "Downloading latest release from GitHub..."
+    curl -s -L -o /tmp/observe-agent.tar.gz https://github.com/observeinc/observe-agent/releases/latest/download/observe-agent_Linux_$(arch).tar.gz
+    ZIP_DIR="/tmp/observe-agent.tar.gz"
+else
+    echo "Installing from provided zip file: $ZIP_DIR"
+fi
+
+# Set up the temp dir and extract files.
+rm -rf $tmp_dir
+mkdir -p $tmp_dir
+tar -xzf $ZIP_DIR -C $tmp_dir
+
+# Create the needed directories.
+echo "Creating system install folders. This may ask for your password..."
+sudo mkdir -p $observeagent_config_dir /var/lib/observe-agent/filestorage
+sudo chmod +rw /var/lib/observe-agent/filestorage
+
+# Move the binary to the proper path.
+cp -f $tmp_dir/observe-agent $agent_binary_path
+
+# Copy all config files to the proper dir.
+sudo cp -f $tmp_dir/otel-collector.yaml $observeagent_config_dir/otel-collector.yaml
+sudo rm -rf $observeagent_config_dir/connections
+sudo cp -fR $tmp_dir/connections $observeagent_config_dir/connections
+sudo chown -R root:root $observeagent_config_dir
+
+# Set up the systemd service if it doesn't exist already.
+if ! systemctl list-unit-files observe-agent.service | grep observe-agent >/dev/null; then
+    echo "Installing observe-agent.service as a systemd service. This may ask for your password..."
+    sudo cp -f $tmp_dir/observe-agent.service /etc/systemd/system/observe-agent.service
+    sudo chown root:root /etc/systemd/system/observe-agent.service
+    sudo systemctl daemon-reload
+    sudo systemctl enable observe-agent.service
+    sudo systemctl start observe-agent
+elif systemctl is-active --quiet observe-agent; then
+    echo "Restarting observe-agent.service. This may ask for your password..."
+    sudo systemctl restart observe-agent
+fi
+
+# Initialize the agent config file if it doesn't exist.
+if [ -f "$observeagent_config_dir/observe-agent.yaml" ]; then
+    echo "Leaving existing observe-agent.yaml in place."
+else
+    echo "Initializing observe-agent.yaml"
+    sudo $agent_binary_path init-config --token $TOKEN --observe_url $OBSERVE_URL --config_path $observeagent_config_dir/observe-agent.yaml
+    sudo chown root:root $observeagent_config_dir/observe-agent.yaml
+fi
+
+echo "Observe agent successfully installed to $agent_binary_path with config in $observeagent_config_dir"