[Heartbeat] Add support for bearer_token_file for HTTP monitors

[paulb-elastic]

Metricbeat has the bearer_token_file option that will read the contents of the file once at initialization and then use the value in an HTTP Authorization header, that will also be useful in Heartbeat for lightweight monitors.

The use case defined is initially requested (internally, via slack and Jira) to be able to configure this via the Elastic Agent YML configuration (not project monitors or via the UI). Reminder that with this type of use case it's intended for Uptime to be used.

ACs:

• Add support for the bearer_token_file configuration for HTTP monitor types
  • This will require an update to the Synthetics integration package
• If defined, include the authorization header Authorization: Bearer [file contents] in all HTTP lightweight requests
• This must not impact any other headers that are also configured for the monitor

[elasticmachine]

Pinging @elastic/uptime (Team:Uptime)

[paulb-elastic]

Pushing this to the backlog for the time being as we're not able to prioritise this infrastructure monitoring (Heartbeat) use case right now.

[botelastic]

Hi!
We just realized that we haven't looked into this issue in a while. We're sorry!

We're labeling this issue as Stale to make it hit our filters and make sure we get back to it as soon as possible. In the meantime, it'd be extremely helpful if you could take a look at it as well and confirm its relevance. A simple comment with a nice emoji will be enough :+1.
Thank you for your contribution!