Move notifications-core into its own repo

[dblock]

Is your feature request related to a problem? Please describe.

The notifications repo is the only one in OpenSearch plugins that produces 2 plugins, notifications and notifications-core. This breaks some assumptions made by the distribution. For example, we had to do a hack (and as a consequence notifications build scripts cannot be checked into the notifications repo) in opensearch-project/opensearch-build#1959, and I suspect we will have to do more. Supporting multiple artifacts was attempted in opensearch-project/opensearch-build#1956 but had other drawbacks.

Describe the solution you'd like

It seems like the easiest solution to avoid these hacks is to put notifications-core into its own project and repo.

Alternatively we should probably go finish the work in opensearch-project/opensearch-build#1956.