[ResponseOps][MW] Allow solutions to extend the scope of the MW

[cnasikas]

The MW client and public APIs allow you to configure the scope of the MW. At the moment, the only supported scope where the MW is applied is the MW. We need to allow users to extend the scope of the MW with their own schema to be able to support applying the MW on their own workflows.

DoD

• Create a backend MW registry where solutions define their scope schema.
• The solution's scope schema will extend the scope schema of the MW public APIs.
• The solution's scope schema will extend the scope schema of the MW client.
• The MW client will persist and retrieve the solution's scope.
• If a solution register a new scope or change an existing schema a test that the RO owns will fail. Solutions will need to update the test and the RO team will get pinged as code owners.

[elasticmachine]

Pinging @elastic/response-ops (Team:ResponseOps)