Add per-package `coverage` enforcement mechanism

@pdhotems has requested that we enforce a minimum code coverage number, below which we will deny the pullrequest.

I didn't receive any specific feedback on the feature itself. I think the way I'd _want_ this to work is to offer a `pyproject.toml` customization that will enable this behavior. That way nobody will be surprised.

Here is a section of the `azure-ai-ml` `pyproject.toml`

```toml
# existing config
[tool.azure-sdk-build]
mypy = true
pyright = false
type_check_samples = false
pylint = true
mindependency = false
latestdependency = false
black = true
# new keys
enforce_coverage = true
coverage_limit = 0.3 # the check will be <=
```

Where the overall coverage goal comes from `coverage_limit`, and `enforce_coverage` enables this behavior. Opt-in to start.

まず、azure-ai-ml pyproject.toml [tool.azure-sdk-build] の設定がどのように読み取られるか、また pull request のチェックでカバレッジがどこで計算されるかを確認します。enforce_coverage と coverage_limit のオプトイン動作を定義し、カバレッジが設定された上限以下の場合に何が起こるかを含めたうえで、リポジトリに既存のチェックテストを使用して動作を検証します。

索引モデルが issue の本文から書いたものです。