[Fleet] Create API endpoint to set fleet.timeout across all policies

[kpollich]

Ref elastic/elastic-agent#2257
Ref elastic/elastic-agent#2408

In order to support self-managed proxy users who may not be able to control their proxies' timeout configurations, Fleet needs to expose an endpoint that allows them to limit Fleet's long polling timeout to 5 minutes. As part of the above issue/PR, the long polling timeout is being increased to 10 minutes by default for all agents. Users who rely on a proxy with a 5 minute timeout will need a way to revert this change on an opt-in basis.

Create an API + persisted setting, e.g

POST /api/fleet/settings
{
  "agent_check_in_long_poll_interval": "5m|10m" // Only options are 5 or 10 minutes
}

When this setting is updated, all policies need to be redeployed, and each policy generated should include the appropriate fleet.timeout value. See the docs in the above PR for an example:

https://github.com/elastic/elastic-agent/pull/2408/files#diff-f0d296ac94b8457b06ae3376b0642cb39fea46a5c630bcca24701d3464b28b5bR47-R54

[elasticmachine]

Pinging @elastic/fleet (Team:Fleet)

[kpollich]

This can be handled via the Fleet settings framework now, afaik.

[jlind23]

@kpollich shall we change this estimate to an S then?