Publish semver-stable 1.0

[alexeagle]

Follow-up to
#2572 (comment)

The main README https://github.com/bazel-contrib/rules_go#overview says:

The rules are in the beta stage of development

However we all know that many users are taking a production dependency on rules_go and rely on the ability to upgrade. It would be nice to create a 1.0 milestone to collect the blocking issues/PRs for breaking changes the maintainers have in mind.

Note that rules_python just went 1.0.

[fmeum]

@tyler-french @linzhp

Just brainstorming:

• We may want to drop goos/goarch and go_cross_binary in favor of more generic approaches such as rule extensions and with_cfg.bzl.
• We need to revisit command-line flags and how they interact with rule attributes.
• We should reconsider what we consider part of the public Go context API as well as our provider layout.