Dynamic Discovery Matchers for Servers

[marcoandredinis]

Context #25494

The DiscoveryService monitors cloud/kube resources and merges them as Teleport resources.
It uses a static configuration that is present in the discovery_service section of the teleport.yaml used by the teleport process.

This PR adds a watcher to the discovery_service that will monitor changes in DiscoveryConfig resources.
Every time a new DiscoveryConfig is created/updated, the service will update its internal matchers to include those coming from the DiscoveryConfig.
When a DiscoveryConfig is deleted, the corresponding matchers are removed as well.

To reduce the scope, we are only focusing on the Server matchers.
Database matchers is already implemented (in #33472)
Follow up PRs will include other resource types.

It always starts the 3 Server Matcher, even if no static configuration exists.
When a new Poll (default is every 5 minutes) starts, it will iterate over all the fetchers:

• static configuration (aka discovery_service from teleport.yaml)
• list of dynamic fetchers (updated by the DiscoveryConfig watcher)

[github-actions]

The PR changelog entry failed validation: Changelog entry not found in the PR body. Please add a "no-changelog" label to the PR, or changelog lines starting with changelog: followed by the changelog entries for the PR.

[marcoandredinis]

@strideynet @mdwn Can you please take a look when you get a chance?

[public-teleport-github-review-bot]

@marcoandredinis See the table below for backport results.

Branch	Result
branch/v14	Create PR