[Connector API] Remove dead serialisation code for connector secrets endpoints

[jedrazb]

Changes

Connectors APIs transport actions are always executed locally, which means that the transport requests and responses are never serialised over the wire between cluster nodes.

This PR removes the serialisation dead code for connector secrets endpoints and logic (1 out of 3 PRs to remove dead serialisation code from API implementation)

This PR follows the approach from this PR #105497 see more context in following issue: #100111

Remove all serialisation tests

Verification

• Pipeline is green
• Verified locally

[elasticsearchmachine]

Pinging @elastic/search-eng (Team:SearchOrg)

[elasticsearchmachine]

Pinging @elastic/search-extract-and-transform (Team:Search - Extract & Transform)

[artem-shelkovnikov]

Can you add more info about serialisation? Is there any doc that can be read about it? Why was it added before?

[jedrazb]

Can you add more info about serialisation? Is there any doc that can be read about it? Why was it added before?

@artem-shelkovnikov this Github issue describes the situation pretty well: #100111.

Context:

• @jimczi pointed out that Connector APIs are executed locally on the node receiving the request so there is no need for serialisation by wire

Why was serialisation by wire added?

• Honestly I was pretty new to ES when implementing this and I just followed pattern from query rules implementation that implements this logic
• The transport action superclass enforces the request/response action to implement Writeable interface, I was not aware that for local actions we can use TransportAction.localOnly(); as a workaround