Extract dashboards-notifications and notifications onto separate repos

[dblock]

Is your feature request related to a problem?

Coming from opensearch-project/opensearch-build#2188, it's not possible to release and tag notifications separately from dashboards-notifications as, for example, 2.0.0. This means we must release the two plugins at the same time.

What solution would you like?
The plugins should live in their own repos.

What alternatives have you considered?
A tagging scheme such as opensearch-2.0.0 vs. opensearch-dashboards-2.0.0.

[dblock]

See opensearch-project/opensearch-build#2188 (comment) in which we decided to split front-end and back-end components in v2.5.0.

[lezzago]

This has been completed