[FEAT] add getInstallationUrl method

[rpmccarter]

Resolves #541

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Before the change?

• determining the installation url for the github app requires searching the docs and finding both the required installation url and the GET /app endpoint for getting the metadata for the current app

After the change?

• the app.getInstallationUrl({ state, target_id }) method crafts the installation url for the user

Pull request checklist

• Tests for the changes have been added (for bug fixes / features)
• Docs have been reviewed and added / updated if needed (for bug fixes / features)

Does this introduce a breaking change?

Please see our docs on breaking changes to help!

• Yes
• No

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Don't love how getInstallationUrl looks so similar to getInstallationOctokit but "installation" means something different. octokit/oauth-methods.js uses getWebFlowAuthorizationUrl but getWebFlowInstallationUrl seemed a little unnecessarily long

The other caveat is that the base url gets cached, so it won't update if someone updates the name of their github app while the program is running - not sure how common github app renames are

[github-actions]

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[gr2m]

Question: how is the state argument used? Is it passed to the post installation setup URL? I've never used it ... is that documented somewhere?

[rpmccarter]

@gr2m yep, the state argument is preserved and passed to subsequent redirect urls (link to the docs). In my case, we request user oauth authorization during our installation step, so the state gets passed in the oauth redirect url

[gr2m]

This is a great PR, thank you for the additional clarification in the README. Just one comment

[rpmccarter]

@gr2m is there anything left to do on my side to get this merged? I'm unable to merge myself as I don't have write access

[github-actions]

🎉 This PR is included in version 15.1.0 🎉

The release is available on:

• GitHub release
• npm package (@latest dist-tag)

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