[data.search.session] Change search session client to implement plugin

[lukasolson]

Summary

This PR changes the way that the data plugin search service implements the session service. Prior to this PR, the session service didn't implement the plugin lifecycle (in either the OSS or enhanced versions).

In doing so, it also adds the cancel logic to the session service, which in the enhanced version, will actually remove it from the search mapping.

This paves the way to add additional methods directly on the session service, like extend (see #87370).

/begin braindump

The OSS session service/x-pack background session service can’t follow the normal plugin lifecycle/types. Here’s why:

1. Data plugin search service gets created, instantiates session service
2. In search service setup, it calls setup of session service
3. Data enhanced plugin gets created, instantiates background session service
4. In enhanced search service setup it calls setup of background session service
5. Enhanced data plugin calls enhance with new background session service

I don’t really like the pattern of “enhancing” using a whole service, since it makes handling the lifecycle really funky… You have to call setup in the enhanced plugin, but then do you call stop on OSS when enhancing? And do you call stop in OSS or enhanced for the enhanced plugin?

I guess all it really means is the enhanced session service can’t extend OSS session service.

/end braindump

Checklist

• Documentation was added for features that require explanation or tutorials
• Unit or functional tests were updated or added to match the most common scenarios

[lukasolson]

@lizozom @Dosant I'd like to hear your thoughts on this approach before cleaning up this PR (fixing tests, types, etc.) I know it seems a bit funky but I can't think of better alternatives at this point.

[kibanamachine]

⏳ Build in-progress, with failures

• continuous-integration/kibana-ci/pull-request
• Commit: 0d0b87c
• This comment will update when the build is complete

Failed CI Steps

• Jest Unit Tests
• X-Pack Jest Unit Tests
• Execute xpack-kibana-ciGroup3
• Check Doc API Changes
• Check Types

History

• 💔 Build #96036 failed 9c04265

To update your PR or re-run it, just comment with:
@elasticmachine merge upstream

[lukasolson]

Superseded by #87966.