signet as Faraday middleware?

With the switch to Faraday, it might make sense to offer signet as Faraday middleware rather than acting as a front-end to it. E.g.

```
client = Signet::OAuth2::Client.new...
access_token = ... # get access token

conn = Faraday.new do |builder|
builder. user Signet::OAuth2::Client, access_token
end

conn.get(...)
```

(Copied from @sqrrrl's issue on the original Signet project.)

Start by reviewing the existing Signet::OAuth2::Client and the Faraday middleware integration described in the example. Define the middleware interface and verify that a Faraday connection can use the OAuth2 access token for requests; the issue names no files or tests to run.

Written by the indexing model from the issue text.