Add support for OAuth2 authentication

[rnd-debug]

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Enhancement request

I would like httpie to support OAuth 2.0 authentication, ideally in a way similar to the infamous P**man. As an example for a client credentials request, users of httpie would supply the info needed to obtain a token (eventually via a config file).

I am aware of the existence of httpie-oauth1 plugin, which hasn't been updated in a while and supports only OAuth 1.0.

I am not sure if this feature request shouldn't be in the httpie/desktop repo.