Dynamic Discovery Matchers for Databases

[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 Database matchers.
Follow up PRs will include other resource types.

It always starts the Database 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)

[marcoandredinis]

@lxea @ryanclark Friendly ping 🙏

[public-teleport-github-review-bot]

@marcoandredinis See the table below for backport results.

Branch	Result
branch/v14	Create PR