[PROPOSAL] Document the process of adding a new plugin to the default distribution

[dblock]

What/Why

What are you proposing?

Document a process for adding a new plugin to the OpenSearch or OpenSearch Dashboards distribution, or releasing a new plugin that is not part of the distribution.

What users have asked for this feature?

Anyone authoring a new plugin .

What problems are you trying to solve?

I was asked whether a hypothetical plugin can be added to a release.

What is the developer experience going to be?

I think the documented process should spell out the following.

1. Pros and cons of being in the distribution, in general.
2. Deciding whether we want a plugin in the distribution, by opening an issue in the plugin repo and first having maintainers of the plugin argue pros/cons of being in the distribution.
3. Deciding which version of the distribution to target and when.
4. Requirements during the plugin development, e.g. gradle build, CI/CD, bcw tests, integ and performance tests.
5. How to onboard into automation, such as version increments.
6. How to onboard into the release manifest in opensearch-build.
7. What is required in terms of security review

cc: @macohen

[macohen]

I started this gist for collaboration: https://gist.github.com/macohen/e7c1b6ae76e3045572a836f767a3ee3d