Add target support of a GitHub PR

[ericwb]

Is your feature request related to a problem? Please describe.
It would be beneficial to not only target a GitHub repo, but a specific PR. Precli could analyze just the files changed in the PR for analysis.

Describe the solution you'd like
Reuse the GitHub target extension to support a PR URL.

Describe alternatives you've considered
n/a

Additional context
n/a

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