Add support for specifying the severity of a SARIF Result

* Extend SarifResult with level property, and allow rule configuration
* Create SarifReportingConfiguration which allow configuring rules with
a default priority, severity and an enable-toggle
* Support for setting the level property[1] of a result.

If unset, it defaults to "warning", which is the result of an empty
default configuration on rules[2]

[1]: https://docs.oasis-open.org/sarif/sarif/v2.1.0/os/sarif-v2.1.0-os.html#_Toc34317648
[2]: https://docs.oasis-open.org/sarif/sarif/v2.1.0/os/sarif-v2.1.0-os.html#_Toc34317855

Differential Revision: https://reviews.llvm.org/D131084