demo: add Mastodon OAuth demo app

[ThisIsMissEm]

There were quite a few things that we're quite working right in OAuth4Swift for non-atproto use cases, see the changesets for the details.

PRs that are part of this branch, but exist separately:

• fix: include client_id in authorization URL when not using PAR #48
• feat: Improvements to AuthServerMetadata #49
• feat: add ClientAuth.Method and SecretMethod protocols #50
• feat: add token revocation (RFC 7009) #51
• feat: gate refresh token requests on grant_types_supported (RFC 8414) #52

[changeset-bot]

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Latest commit: df8d21f

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[ThisIsMissEm]

n.b., keychain likely isn't the correct primitive here since it outlasts an application uninstall and reinstall. I just don't know what the preferred storage for sensitive data is that is actually bound to the application install.