[deploy_maven] allow for release repo override via command line

[sugarmanz]

What is the goal of this PR?

Allow for the release repo to be configured dynamically. With Maven Central, there is the opportunity to batch a release with a singular staging repository. Unfortunately, without some rework, it would be a little difficult to dynamically set the release repository. I don't really want to dynamically touch the BUILD or .bzl files where I might have the release_repo defined that is passed into the rule since that could differ between repos. This change allows the release_repo to be overridden from a CLI such that the staging repository can be created dynamically during CI and passed to the deploy target for publishing before dynamically closing & releasing the staging repo.

Related shell script

What are the changes implemented in this PR?

This is an interesting PR b/c there are a few ways to solve this. I could have modified the .py script to try and read from an env var. However, given that this is really a one-off URL, the potential for a stale repo being stored and used from the env was non-zero. Having to manually specify the staging repo w/in the bazel invocation seemed like a good middleground.

Opinions appreciated 😄

[alexjpwalker]

I agree with the approach of using --define! This is aligned with how we override the version being deployed:

bazel run --define version=$(cat VERSION) //:deploy-maven -- release

A couple of points:

1. Since we're allowing users to override the release repo, we should also allow them to override the snapshot repo.
2. As version is all lowercase, so should be deploy_maven_release_repo; and for terseness, I'd call it just release_repo. (As this is a run target, the key need not be globally unique.)

With all that in mind, and simplifying the Starlark a little:

Suggested change

	release_repo = ctx.attr.release
	if _DEPLOY_MAVEN_RELEASE_REPO_KEY in ctx.var:
	release_repo = ctx.var[_DEPLOY_MAVEN_RELEASE_REPO_KEY]
	snapshot_repo = ctx.var.get("snapshot_repo", default=ctx.attr.snapshot)
	release_repo = ctx.var.get("release_repo", default=ctx.attr.release)

(requires additional update to L330: "{snapshot}": snapshot_repo,)