Add Code Coverage GH action

[Alirexaa]

Pull Request

Related issue

Fixes #507

What does this PR do?

This pull request introduces a new GitHub Actions workflow for measuring and reporting code coverage. The workflow is configured to run on specific branches and includes steps to set up the environment, install necessary tools, and upload coverage reports to Codecov.

Key changes:

• Added a new GitHub Actions workflow named coverage.yml to measure code coverage on trying, staging, and main branches.
• Configured permissions to allow reading contents and writing issues.
• Set up the environment to use Nightly Rust and installed cargo-llvm-cov for coverage data collection.
• Added steps to run Meilisearch using Docker, collect coverage data, and upload the reports to Codecov.

PR checklist

Please check if your PR fulfills the following requirements:

• Does this PR fix an existing issue, or have you listed the changes applied in the PR description (and why they are needed)?
• Have you read the contributing guidelines?
• Have you made sure that the title is accurate and descriptive of the changes?

Thank you so much for contributing to Meilisearch!

[Alirexaa]

@brunoocasali, I ran this action in my fork.
You can see coverage results at https://app.codecov.io/github/Alirexaa/meilisearch-rust/pull/1/tree.

Make sure CODECOV_TOKEN has been set in repo secrets.