DiscoveryConfig: Add status field to report discovered objects and …

…errors (#40136)

* DiscoveryConfig: Add `status` field to report discovered objects and
errors

This PR introduces a `status` field into the `discovery_config` protobuf
spec to report status and metrics about the resources discovered for
each `discovery_config` spec.

Signed-off-by: Tiago Silva <tiago.silva@goteleport.com>

* add last_sync_time

* fix typo

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Signed-off-by: Tiago Silva <tiago.silva@goteleport.com>