Use custom headers to communicate server/API versions

[Flix6x]

• All API responses should include the FlexMeasures-Version: 0.32.0 header, updated with the current server version.
• All API responses under the v3_0 blueprint should also include the API-Version: v3_0 header.
• See flexmeasures.api.get_versions (in __init__.py) for some implementation details.
• Add tests
• Create a corresponding PR in https://github.com/FlexMeasures/flexmeasures-client, noting related work in:
  • Feat/check server version flexmeasures-client#119
  • Add minimum_server_version parameter to client.request flexmeasures-client#151