feat: add optional version arg to install scripts

[obs-gh-mattcotter]

Description

OB-40308 add optional version arg to install scripts. This will allow our old Host Quickstart app installation instructions to be pinned to an older version as we update the agent.

Example usage:

PS C:\Users\Administrator\Downloads> .\install.ps1 -version 1.4.0

PS C:\Users\Administrator\Downloads> & 'C:\Program Files\Observe\observe-agent\observe-agent.exe' version
Using config file: C:\Program Files\Observe\observe-agent\observe-agent.yaml
observe-agent version: 1.4.0

$ bash ./scripts/install_mac.sh --version 1.5.0
Downloading version 1.5.0 from GitHub...
# ...

$ observe-agent version
observe-agent version: 1.5.0
observe-agent config file: /usr/local/observe-agent/observe-agent.yaml

Checklist

• Created tests which fail without the change (if possible)
• Extended the README / documentation, if necessary