Initial work towards supporting Groups v2

[gferon]

I open this PR so we can start discussing about the general direction we want to take for this change.

Expect: dirty code, hacks, ugly things and... being able to fetch the metadata of a group v2 given that you posses its master_key (which is part of what the client receives on every new message).

I'd mostly like to get some early review, and answers to the following questions:

• Is the trait for CredentialsCache ok? (It's modeled after the cache used in the Java source code)
• awc::Client sets the Authorization header twice if it's set at the client and request level, so I had to remove it from get_client and set it manually everywhere. This is error prone, but I couldn't think of another way
• Are there too many .unwrap() in this change? 💝

TODO:

• Decrypt the incoming data
• Clean things up

[rubdos]

Just some things that caught my eye.

[rubdos]

Doesn't look to bad tbh.

Maybe think about the api.rs naming, because everything is API in the whole crate.

[rubdos]

empty review, that's not useful.

[gferon]

Doesn't look to bad tbh.

Maybe think about the api.rs naming, because everything is API in the whole crate.

What about having everything in mod.rs? It's really not that much code.

[rubdos]

Doesn't look to bad tbh.
Maybe think about the api.rs naming, because everything is API in the whole crate.

What about having everything in mod.rs? It's really not that much code.

That, or modeled after AccountManager we could have group_manager.rs?

[rubdos]

Some last very minor nitpicks. Thanks for the work!