[RAM] Add new privilege for observability serverless

[XavierM]

In order to enable the Elasticsearch query and generic threshold to function effectively in an observability serverless project, we want to add a new Kibana feature privilege specifically tailored for observability within serverless environments. This new privilege will encompass all rule types accessible to users within the serverless project.

To ensure this observability feature is only registered in serverless projects, we intend to introduce a new configuration attribute within the observability YAML serverless config file. Consequently, when APM and SLO register their respective rule types within a serverless context, they will need to update their producer to be classified under "observability."

Our next step will involve modifying the new scope selector to exclusively utilize the observability feature and it will be hidden from our users' view. This adjustment will be guided by an API that informs us about the feature access associated with our rule types, providing us with the necessary insights to fine-tune our scope settings accordingly.

[elasticmachine]

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