Add support for more parameters when using oauth2

[rvangsgaard]

In order for oauth2 to succeed in our Azure AD setup, we must specify the resource parameter when getting bearer token. Other users might have the same issue.

I have tried to add the capability of adding payload parameters in the "get token" request in a PR #2047

[devantler]

I found a few issues related to the lack of payload parameters for the OAuth2 request:

#1812
#1213

I hope this might raise awareness as it currently limits the possibility of using Benthos. 🙏🏻