[FR] Simple validation of stamp substitutions in expand_template

[njlr]

It would be useful to be able to provide simple validations of values given to expand_template to ensure that incorrect information has not been given to Bazel. If the values do not pass validation, the build should fail.

This is particularly important when implementing "deliverables", since an incorrect stamp might lead to artifacts going to the wrong place.

A validation regex seems like a good balance of simplicity / expressiveness.

For example, a sem-ver regex: https://semver.org/#is-there-a-suggested-regular-expression-regex-to-check-a-semver-string