[Feature Request] Allow specifying commit message in params

[tanner-bruce]

Currently you can only set the commit message in the sourceblock. We ran in to a case where following what is going on in the pipeline would be simplified by being able to set the commit message in params on the put step.

[lockan]

+1 to this request. Allowing the commit_message in the params would make more semantic sense, and would also allow for pipelines where multiple version bumps on the same repo might be required with different messaging.

[vito]

FWIW, concourse/rfcs#24 completely eliminates the difference between source and params from a resource's perspective so we should get things like this for free in the future.