[Metrics] metrics_data_access plugin (#164094)

## Summary

Closes https://github.com/elastic/kibana/issues/161876

Creates a plugin providing utilities to access metrics data. The plugin
only exposes a server API which includes a client with two methods:
- `getMetricIndices` to retrieve the user-defined indices where metrics
are located
- `updateMetricIndices` to update the indices

The client is now used where we previously relied on infra plugin to
provide the configuration, in APM and Infra.

The plugin persists the configuration in a new saved object
`metrics-data-source`. Because this configuration was previously stored
in the `infrastructure-ui-source`, the plugin relies on a fallback to
reuse any existing value (see additional context
https://github.com/elastic/kibana/issues/161876#issuecomment-1673537400).

### Reviewers
There are no functional changes outside of Infra Monitoring UI and APM
UI, other codeowners are involved because this introduces a new saved
object
- APM - the change introduces a drop-in replacement of the
`infra.getMetricIndices` call. The ui code still relies on infra plugin
for a couple of components so we can't drop the dependency yet, those
we'll need to be moved to a tier 2 plugin (more details in
https://github.com/elastic/observability-dev/discussions/2787
(internal)) in a separate issue

### Testing
You'll need metrics data to verify data fetching works (I've used an
edge-oblt cluster)

1. Navigate to Infrastructure Settings and verify metric indices are
configured with the default value of `infrastructure-ui-source`
2. Update metric indices settings (if connected to oblt cluster add
`remote_cluster:..` indices)
3. Verify `metrics-data-source` saved object is persisted with correct
attributes
4. Verify Infrastructure Inventory is pulling data from the newly
configured indices
5. Go to APM services, verify service Infrastructure pulls data from
newly configured indices

---------

Co-authored-by: kibanamachine <42973632+kibanamachine@users.noreply.github.com>
Co-authored-by: Jason Rhodes <jason.rhodes@elastic.co>

docs/developer/plugin-list.asciidoc

@@ -648,6 +648,10 @@ using the CURL scripts in the scripts folder.
 |Visualize geo data from Elasticsearch or 3rd party geo-services.
 
 
+|{kib-repo}blob/{branch}/x-pack/plugins/metrics_data_access/README.md[metricsDataAccess]
+|Exposes utilities to access metrics data.
+
+
 |{kib-repo}blob/{branch}/x-pack/plugins/ml/readme.md[ml]
 |This plugin provides access to the machine learning features provided by
 Elastic.