Follow-up for elastic/package-spec#351

Once Elastic Package Registry (EPR) has added support for content packages that contain discovery properties:

discovery:
  fields:
    - name: process.pid

the package registry should also be able to index and search packages based on the information included in the discovery property of each package.

For example the following request to EPR would return packages that can leverage documents that include the process.pid or the user.id fields.

GET /search?discovery=fields:process.pid,user.id