Add CA fingerprint to agents configuration in 8.x versions

[agithomas]

The request is to support stand along agents connect with elastic stack created using elastic-package stack up over https.

These setting is needed in the default profile . This comes to use only when --certificate-authorities=$ELASTIC_PACKAGE_CA_CERT flag is present when adding the agent.

The configuration goes as below

xpack.fleet.outputs:
  - id: fleet-default-output
    name: default
    type: elasticsearch
    hosts: [ https://elasticsearch:9200/ ]
    # openssl x509 -fingerprint -sha256 -noout -in tls/kibana/elasticsearch-ca.pem (colons removed)
    ca_trusted_fingerprint: <fingerprint generated using $ELASTIC_PACKAGE_CA_CERT>
    is_default: true
    is_default_monitoring: true

[agithomas]

Since the default output is managed, in kibana there exist no way to add the fingerprint through kibana making the above configuration essential

I have tested using 8.5.1 stack and agent.

[agithomas]

Changes in the documentation in elastic-package is recommended as the console where elastic-package stack up command ran which internally runs eval "$(elastic-package stack shellinit)" may be different from the console where the user try to add the agent using the command

sudo ./elastic-agent install --certificate-authorities=$ELASTIC_PACKAGE_CA_CERT --fleet-server-es-ca=$ELASTIC_PACKAGE_CA_CERT --url=https://fleet-server:8220/ --enrollment-token=<enrollment token>