Relates elastic/package-spec#919

Add a new mechanism to discover content packages. This mechanism should be based on datasets.

Packages define this new mechanism in the package manifests as:

discovery:
  fields: []
  datasets:
    - name: nginx.access
    - name: nginx.error

Example of query:

GET /search?discovery=datasets:nginx.access,hostmetricsreceiver.otel

Package Registry service should return all the packages that define at least one of the datasets present in the query parameter. For instance, given the query from above, these packages would be part of the response:

discovery:
  fields: []
  datasets:
    - name: nginx.access
    - name: nginx.error

discovery:
  fields: []
  datasets:
    - name: hostmetricsreceiver.otel
    - name: other.dataset

Some requirements for these queries related to discovery of packages:

• Queries based on discovery.fields:
  • All discovery fields in the package manifest must appear in the query parameter

  GET /search?discovery=fields:host.ip,event.dataset:nginx.other`
  

• Queries based on discovery.datasets:
  • At least one discovery dataset in the package manifest must appear in the query parameter:

  GET /search?discovery=datasets:nginx.other` will return packages based on `discovery.datasets` values
  

• Queries to use both mechanism to discovery packages (multiple discovery parameters):
  • Both conditions for discovery must be fulfilled (fields and datasets).

  GET /search?discovery=datasets:nginx.other&discovery=fields:host.ip`