Add support for sending build failure notifications to different CODEOWNERS owners than the origin build definition owners.

[konrad-jamrozik]

We interpret CODEOWNERS file to route build failure notifications, as described here. Specifically, if given CODEOWNERS path matches to build definition like ci.yml, the CODEOWNERS owners will be notified of build failures of this build definition.

Because CODEOWNERS is also used to determine who is assigned to PR reviews, as a result, the set of PR reviewers for given build definition is the same set of people who get notified about that build definition build failures.

However, some of the time we don't actually want that: we want for different people to review the given build definition changes than be notified of its build failures. For an example of this, see this comment and surrounding comments:

• Remove CODEOWNERS rules /**/ci.yml and /**/tests.yml azure-sdk-for-net#33595 (comment)

This work is about adding this capability.

We will make the notification-configuration tool search for owners not of the pipeline definition file it originated from (e.g. /foo/ci.yml), but a made-up sibling file named PipelineOwner, e.g. /foo/PipelineOwner. This way if there is a CODEOWNERS path that matches to /foo/ci.yml it is not going to be used to determine where to route build failure notifications, but as a CODEOWNERS path that matches /foo/PipelineOwner, e.g. /foo/, will. You can find the details of this approach, and preceding discussion, here:

• Remove CODEOWNERS rules /**/ci.yml and /**/tests.yml azure-sdk-for-net#33595 (comment)

This work is to be picked up during the same time frame as:

• Implement CODEOWNERS file changes validation (gating), with baselining #4859

Obsolete approach

Below is an obsolete approach which we rejected, per the discussion here:

• Remove CODEOWNERS rules /**/ci.yml and /**/tests.yml azure-sdk-for-net#33595 (comment)

We want to add a comment directive that instructs our CODEOWNERS interpreter to ignore given path for the purpose of determining who to notify about build failures from given build definition.

[konrad-jamrozik]

@weshaggard @benbp instead of doing equivalent of this PR for other language repos:

• Remove CODEOWNERS rules /**/ci.yml and /**/tests.yml azure-sdk-for-net#33595

I am going to work on implementing support for this work item, and then add relevant "ignore for purposes of build failure notification" comment directive to the /**/ci.yml and /**/tests.yml paths in the CODEOWNERS files. If you want to quickly see the contents of all the CODEOWNERS files, you can find links to them here:

• Enable the new, regex-based, wildcard-supporting CODEOWNERS matcher #5088

[konrad-jamrozik]

This work item has been cancelled. For context, see this comment.