Provide missing LicenseDot API at CustomersDot

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### Problem

[VersionsApp](https://gitlab.com/gitlab-services/version-gitlab-com) while processing received [ServicePing](https://docs.gitlab.com/ee/development/service_ping/) payload is reaching out to [LicenseDot](https://gitlab.com/gitlab-org/license-gitlab-com) in order to fetch details about reporting instance license. After deprecation of LicenseDot ServicePing reporting process start to raise errors https://sentry.gitlab.net/gitlab/versiongitlabcom/issues/3209409 This cause ServicePing payloads being recorded without linkage to zuora account, which limits their utility in business analysis. 

Corresponding issue on ~"group::product intelligence" tracker https://gitlab.com/gitlab-org/gitlab/-/issues/354025

### Proposal

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@cwiesner [suggested](https://gitlab.com/gitlab-org/gitlab/-/issues/354025#note_861527955) adding new API endpoint that would provide the missing features from LicenseDot [`LicenseController`](https://gitlab.com/gitlab-org/license-gitlab-com/-/blob/master/app/controllers/licenses_controller.rb)

Missing features:
- [x] Token authentication
- [x] Fetching license by MD5 hash (LicenseDot was returning matching licenses collection, but only [first](https://gitlab.com/gitlab-services/version-gitlab-com/-/blob/master/app/services/license_app_service.rb#L43) was used at VersionsApp)
- [x] Fetching license by id 

### Result

- [x] Missing API is provided
- [x] Auth token for VersionsApp is issued 

### Next steps (if any)

### How will we measure success?

Communication between VersionsApp and CustomersDot is restored.

### Availability & Testing

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* Integration test changes
* End-to-end test change

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If any code is borrowed from LicenseDot, additionally bring in any associated tests.  Once brought in, we should also consider writing an integration test to ensure the connection between the two applications.

Story points: 2.0