Exempt specific files from Codeowners rules

What does this MR do and why?

Problem

Currently there is no way to exempt specific files from requiring code owner approval.

Solution

This adds support for exclusion patterns in CODEOWNERS using the '!' prefix:

* @default-owner      # All files require approval
!pom.xml              # Except pom.xml which needs no approval

Approach

Given that entries_for_path is where we determine file ownership, we modified Gitlab::Codeowners::Entry to support exclusion patterns by adding an exclusion flag(!). When processing CODEOWNERS patterns in order, patterns starting with ! are marked as exclusions. So during path matching in entries_for_path, if any matching entry is an exclusion, that file is excluded from requiring ownership approval for that section.

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Related to #41914 (closed)