Add support for using external participant and authorization registries (part 2)

Proposed changes

The main part of this PR is implementing experimental support for having an internal authorization registry. This PR also makes some improvements into the external participant code to also support client_credentials flow and client_assertion_type jwt-bearer.

Types of changes

[ ] Bugfix (non-breaking change which fixes an issue)
[x] New feature (non-breaking change which adds functionality)
[ ] Breaking change (fix or feature that would cause existing functionality to not work as expected)

Checklist

Put an x in the boxes that apply. You can also fill these out after creating the PR. If you're unsure about any of them, don't hesitate to ask. We're here to help! This is simply a reminder of what we are going to look for before merging your code.

[x] I have read the CONTRIBUTING doc
[x] I have signed the CLA
[x] I have added tests that prove my fix is effective or that my feature works
[ ] I have added necessary documentation (if appropriate)
[ ] Any dependent changes have been merged and published in downstream modules

Further comments

Continuation of PR #206

ging / fiware-idm