[Infra IU] Disable Metrics Explorer for serverless (#167022)

Closes #163282 

## Summary

This PR:
* Adds a `featureFlags.metricsExplorerEnabled` property to the Infra
plugin config to enable and disable Metrics Explorer depending on the
offering type
* Prevents `MetricsExplorerViewsService` initialization for serveless
based on the feature flag
* Prevents creating Metrics Explorer frontend routes when in serverless
* Prevents registration of the MetricsExplorerViews saved object when in
serverless
* Prevents initialization of the `metrics_explorer_views` API routes
when in serverless

**Trying to access Metrics Explorer in serverless**
<img width="1829" alt="CleanShot 2023-09-22 at 12 59 35@2x"
src="2b039925-0f0b-4c07-be29-bbe910de7a34">

**Trying to access views API**
<img width="1829" alt="CleanShot 2023-09-22 at 13 00 00@2x"
src="15269ec2-becd-4ee3-9b5e-d916df28a7b8">

**`infra/metrics_explorer` API still works as per ticket requirements**
<img width="1829" alt="CleanShot 2023-09-22 at 13 00 06@2x"
src="fb23f912-c6fd-46c8-9084-c17c51e5b064">


## How to test
* Checkout locally
* Enable Infra in `serverless.oblt.yml`: `xpack.infra.enabled: true`
* Run Kibana in serverless mode
* Try accessing `/app/metrics/explorer` route and make sure it's not
available
* Make sure other Infra routes (`/app/metrics/inventory` and
`/app/metrics/hosts`) still load as expected
* In Kibana dev console make sure you get 404 for `GET
kbn:/api/infra/metrics_explorer_views`
* Also check that you don't see `metrics-explorer-view` saved object in
the response for `GET
kbn:/api/kibana/management/saved_objects/_allowed_types`
* Run Kibana in non-serverless mode and make sure Metrics Explorer is
accessible and works as usual

---------

Co-authored-by: Kibana Machine <42973632+kibanamachine@users.noreply.github.com>

test/plugin_functional/test_suites/core_plugins/rendering.ts

@@ -250,6 +250,11 @@ export default function ({ getService }: PluginFunctionalProviderContext) {
         'xpack.index_management.enableLegacyTemplates (any)',
         'xpack.index_management.enableIndexStats (any)',
         'xpack.infra.sources.default.fields.message (array)',
+        /**
+         * xpack.infra.featureFlags.metricsExplorerEnabled is conditional based on traditional/serverless offering
+         * and will resolve to (boolean)
+         */
+        'xpack.infra.featureFlags.metricsExplorerEnabled (any)',
         'xpack.license_management.ui.enabled (boolean)',
         'xpack.maps.preserveDrawingBuffer (boolean)',
         'xpack.maps.showMapsInspectorAdapter (boolean)',