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.