make integrations self-contained

[jrhizor]

• integrations need to be responsible for defining the config spec
• integration images need an interface to produce the config spec
• integrations should fail if the provided config is invalid
• integration tests should fail if validation is not being performed properly or if it can't access the spec
• integrations should be able to optionally convert their config to a different format (currently only required for BQ, but could be used in the future for postgres, others)

[sherifnada]

solved after #528

[cgardens]

@sherifnada - is moving standard source / standard destination into the integration not part of this task? should we have a separate issue for it? Essentially same question as on this PR comment. Probably easies to just discuss in this isse instead of on the merged PR.

[sherifnada]

@cgardens just saw this comment. Why would moving the entire source/destination definition into the image be part of the task? We still need a registry in the core application of sources/destinations and the docker images/tags they use. This is what standard source/destination currently refer to.

[cgardens]

Let's discuss this tomorrow when we're in person.