Multiple remote connection strategy support

[Tim-Brooks]

This commit backports three commits from master.

Extract remote "sniffing" to connection strategy (#47253)

Currently the connection strategy used by the remote cluster service is
implemented as a multi-step sniffing process in the
RemoteClusterConnection. We intend to introduce a new connection strategy
that will operate in a different manner. This commit extracts the
sniffing logic to a dedicated strategy class. Additionally, it implements
dedicated tests for this class.

Additionally, in previous commits we moved away from a world where the
remote cluster connection was mutable. Instead, when setting updates are
made, the connection is torn down and rebuilt. We still had methods and
tests hanging around for the mutable behavior. This commit removes those.

Introduce simple remote connection strategy (#47480)

This commit introduces a simple remote connection strategy which will
open remote connections to a configurable list of user supplied
addresses. These addresses can be remote Elasticsearch nodes or
intermediate proxies. We will perform normal clustername and version
validation, but otherwise rely on the remote cluster to route requests
to the appropriate remote node.

Make remote setting updates support diff strategies (#47891)

Currently the entire remote cluster settings infrastructure is designed
around the sniff strategy. As we introduce an additional conneciton
strategy this infrastructure needs to be modified to support it. This
commit modifies the code so that the strategy implementations will tell
the service if the connection needs to be torn down and rebuilt.

As part of this commit, we will wait 10 seconds for new clusters to
connect when they are added through the "update" settings
infrastructure.

[elasticmachine]

Pinging @elastic/es-distributed (:Distributed/Network)

[Tim-Brooks]

@elasticmachine run elasticsearch-ci/1