[Feature] [API] List pull requests files

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GET /repos/:owner/:repo/pulls/:pull_number/files

GitHub Rev: https://developer.github.com/v3/pulls/#list-pull-requests-files
GitHub example: GET "https://api.github.com/repos/go-gitea/gitea/pulls/10915/files"

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depending on how #11187 make progress I'll start implementing it or waiting for this pull ...

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this is a dublicate of #654