Renames and refactoring for reloadable plugins

[albertzaharovits]

This handles some renames and javadocs for the reload-secure-store-action feature branch to deck it before merging to master. This is required because the scope of the branch was somewhat fluid, and as the work progressed, the names and comments were not illustrative for their purposes. This has mostly ornamental value but it does contains two refactorings, hence the encompassing PR title.

Here is the list with all renames:

NodesReInitRequestBuilder -> NodesReloadSecureSettingsRequestBuilder
NodesReInitRequest -> NodesReloadSecureSettingsRequest
NodesReInitResponse -> NodesReloadSecureSettingsResponse
NodesReInitAction -> NodesReloadSecureSettingsAction
"cluster:admin/reinit" -> "cluster:admin/nodes/reload_secure_settings"
secureStorePassword -> secureSettingsPassword
TransportNodesReInitAction -> TransportNodesReloadSecureSettingsAction
package org.elasticsearch.action.admin.cluster.reinit; -> package org.elasticsearch.action.admin.cluster.node.reload;
ReInitPlugin -> ReloadablePlugin
ReInitPlugin#reinit -> ReloadablePlugin#reload
RestReInitAction -> RestReloadSecureSettingsAction
AbstractClient#prepareReInit -> AbstractClient#prepareReloadSecureSettings
request.param("secureStorePassword" -> request.param("secure_seettings_password"
#updateClientSettings -> #refreshAndClearCache

All services that lend clients (AwsS3Service, AwsEc2Service, GoogleCloudStorageService and AzureStorageService) have the methods client and refreshAndClearCache. This client method creates (and cache) and returns a client (or a releasable wrapper). The refreshAndClearCache method updates the settings from which new clients are built and clears the cache of the old clients.

Besides renames, there is one notable refactoring: ReloadablePlugin#reload now returns an exception on failure instead of a bland false return value. This cascades back to the node response and how it deals with error handling.
The other refactoring is that the method releaseCachedClient (for AwsS3Service and AwsEc2Service is now hidden and exposed through Closeable#close).

Relates #29135

[elasticmachine]

Pinging @elastic/es-core-infra

[tlrx]

Not much to say, I looked at the renamings and it LGTM.

[jaymode]

LGTM. I left a comment for something to handle in a followup.

[jaymode]

why don't we use a SecureString here? I know this is not anything new in this PR so we can handle in a follow up.