feat: add itest_local_environment rule

[rafikk]

This PR introduces itest_local_environment, a rule for declaring a bazel runnable target for local development. It is very similar to itest_service_group but carries an additional attribute for explicitly mapping port labels to stable port numbers.

Unlike itest_service_group, it is intended to be a terminal rule. It does not return _ServiceGroupInfo, meaning it cannot be used as a dependency in service_test or composed with other service groups or local environments. The purpose of the target is strictly interactive local development via bazel run.

The primary motivation for this PR is to avoid the use of Bazel flags for port mapping. While the Bazel flags are very flexible, they have a few downsides:

1. They're very verbose for command-line usage
2. They're not easily shared without storing in bazelrc
3. They can't be stored in bazelrc without a config prefix as the flags then also apply to integration tests
4. Storing in bazelrc often requires a repo-wide coordination of reserved ports

itest_local_environment avoids these problems. It encodes the port reservations in Starlark, and port numbers only need to be unique within a specific environment target. As these are intended for end-users, port collisions across disjoint environments/groups do not cause a problem in practice.

The port_assignments continue to be overridable via Bazel flags.

[chatgpt-codex-connector]

💡 Codex Review

Here are some automated review suggestions for this pull request.

Reviewed commit: 4540fe7f4f

ℹ️ About Codex in GitHub

Codex has been enabled to automatically review pull requests in this repo. Reviews are triggered when you

• Open a pull request for review
• Mark a draft as ready
• Comment "@codex review".

If Codex has suggestions, it will comment; otherwise it will react with 👍.

When you sign up for Codex through ChatGPT, Codex can also answer questions or update the PR, like "@codex address that feedback".