dpf: Harmonize settings naming in data-plane-transfer

[bscholtes1A]

What this PR changes/adds

• Harmonize setting names used by data-plane-transfer
  • edc.transfer.dataplane.token.signer.privatekey.alias becomes edc.transfer.proxy.token.signer.privatekey.alias
  • edc.transfer.dataplane.sync.token.validity becomes edc.transfer.proxy.token.validity.seconds
  • edc.transfer.dataplane.sync.endpoint becomes edc.transfer.proxy.endpoint
  • edc.public.key.alias becomes edc.transfer.proxy.token.verifier.publickey.alias
  • edc.dpf.selector.strategy becomes edc.transfer.client.selector.strategy
• Fix path of the token validation API:
  • previous path was http://myconnector:8181/api/v1/validation/validation
  • new path is http://myconnector:8181/api/v1/validation/token
• Refactor data-plane-transfer module as all stuff in -core was actually only used by the -sync part.

Why it does that

Provide more consistency in the naming of the setting for a given module.

Checklist

• added appropriate tests?
• performed checkstyle check locally?
• added/updated copyright headers?
• documented public classes/methods?
• added/updated relevant documentation?
• added relevant details to the changelog? (skip with label no-changelog)
• formatted title correctly? (take a look at the CONTRIBUTING and styleguide for details)