FR: Mercurial equivalent of git_repository

### Description of the feature request:

It would be great to have a repository rule for Mercurial repositories, similar to the `git_repository`rule.

### Which category does this issue belong to?

External Dependency

### What underlying problem are you trying to solve with this feature?

This would allow us to create BCR modules for repositories that are hosted on Mercurial, such as [gmp](https://gmplib.org/devel/repo-usage)

### Which operating system are you running Bazel on?

_No response_

### What is the output of `bazel info release`?

_No response_

### If `bazel info release` returns `development version` or `(@non-git)`, tell us how you built Bazel.

_No response_

### What's the output of `git remote get-url origin; git rev-parse HEAD` ?

```text

```

### Have you found anything relevant by searching the web?

None. Such a rule does not yet exist.

### Any other information, logs, or outputs that you want to share?

_No response_

Start by locating the existing git_repository rule and its surrounding external-dependency tests to understand the supported repository-rule interface. Then determine the Mercurial fetching behavior needed for BCR modules such as gmp; done means a Mercurial-hosted module can be declared and fetched through the new rule with coverage for expected inputs and failures.

Written by the indexing model from the issue text.