Consider adding new hooks - request_access_token and request_refresh_token

[viperfx]

These hooks are implemented right before the request is sent for the fetch token and similar for the refresh token request too.

These hooks are required for me to apply a compliance fix for Azure Devops

https://docs.microsoft.com/en-us/azure/devops/integrate/get-started/authentication/oauth?view=azure-devops

[jtroussard]

Please feel free to raise a PR with these changes and unit tests.

[violuke]

This should be possible now, see #433