Add access token auth support to Client and other changes

[abevoelker]

I needed the Dwolla client to support OAuth access tokens for a potential partner integration so I made a few changes to accommodate that. I don't expect you to merge this but just wanted to politely let upstream know about my changes in case you wanted to incorporate any.

• Client now accepts a :token parameter that initializes the client with an existing token
• Client will now auto-refresh tokens using the refresh authorization flow (grant_type=refresh_token) in addition to the existing client credentials flow (grant_type=client_credentials)
• Move TokenWrapper functionality into Token, and TokenManager into Client
• Client will gracefully refresh token and retry request if the Dwolla API responds with ExpiredAccessTokenError or InvalidAccessTokenError (in case for whatever reason EXPIRES_IN_LEEWAY was too optimistic)
• Remove eager connection on Client instantiation (was invalidating inflight application authorization requests)

[ShreyaThapa]

Thank you for your contribution to our SDK, @abevoelker! We truly appreciate the time and effort you have put into this PR and apologize for the lack of response and miscommunication on our end. 🙏

We've since changed how partner integrations work in relation to the auth process. Because of this, we are closing this PR and not merging into main. While we are unable to merge this PR, we will make a note of your changes in relation to how this might be better managed with the new auth scheme. In addition, we'll add a ticket to our backlog on how we might apply similar changes to the SDK in the future.