This PR enables RCS 2.0 remote clusters to be configured without the
need to restart nodes. It works as the follows (assuming both clusters
are already running):

1. Get a cross-cluster API key for accessing the _remote_ cluster
2. Add cross-cluster API key to keystores of the _local_ cluster, e.g.

    ```

    echo -n xxx | ./bin/elasticsearch-keystore add cluster.remote.my.credentials -x

    ```

3. Call [ReloadSecureSettings API](https://www.elastic.co/guide/en/elasticsearch/reference/current/cluster-nodes-reload-secure-settings.html) on the _local_ cluster
4. Configure RCS 2.0 remote cluster should now just work for the _local_ cluster, e.g.

    ```

    PUT /_cluster/settings
    {"persistent":{"cluster":{"remote":{"my":{"seeds":["127.0.0.1:9443"]}}}}}

    ```

This PR does **not** include functionality to automatically re-build
connections on secure settings reload. I will add this in a follow up
PR.

The high level technical approach is to maintain a credentials manager
class and use this to attach credentials for connections to remote
clusters. This
[comment](https://github.com/elastic/elasticsearch/pull/102798/files#r1417708553)
also provides more context on some lower level details.

Relates: #98120 Relates:
ES-6764