[FEAT]: Support refreshtokens in OAuth flow

[Kencdk]

Describe the need

While the library currently supports getting access tokens from an OAuth callback, the resulting object doesn't include the refresh token, which allows an application to get a new access token for the user, without having to go through the entire OAuth flow again.

Additionally, the OAuthClient doesn't support a way to get an access token from a refresh token.

The refresh token is returned in the original request to get an access token.
https://docs.github.com/en/apps/creating-github-apps/authenticating-with-a-github-app/generating-a-user-access-token-for-a-github-app
https://docs.github.com/en/apps/creating-github-apps/authenticating-with-a-github-app/refreshing-user-access-tokens

Proposal:
Add RefreshToken as a property on Octokit/Models/Response/OauthToken.cs
Add RefreshToken as a property on Octokit/Models/Request/OauthTokenRequest.cs

If a OauthTokenRequest contains a RefreshToken but the AuthToken is null only the refresh_token property should be added to the request body due to the null check in RequestParameters.cs::ToParametersDictionary()

SDK Version

No response

API Version

No response

Relevant log output

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Code of Conduct

• I agree to follow this project's Code of Conduct

[Kencdk]

if the proposal is accepted, I'll gladly implement it :)

[kfcampbell]

@Kencdk that seems reasonable to me! PRs are always appreciated.