Add token fetch callback

[ramsayleung]

Description

Add a user-customized callback function support which is invoked whenever client succeeds to request or refresh a token, so it provides users an opportunity to save token into file, store into database etc.

Motivation and Context

400

Dependencies

None

Type of change

Please delete options that are not relevant.

• [ ] Bug fix (non-breaking change which fixes an issue)
• [x] New feature (non-breaking change which adds functionality)
• [ ] Breaking change (fix or feature that would cause existing functionality to not work as expected)
• [x] This change requires a documentation update

How has this been tested?

• [x] with_token_callback_fn.rs

Is this change properly documented?

Please make sure you've properly documented the changes you're making.

Don't forget to add an entry to the CHANGELOG if necessary (new features, breaking changes, relevant internal improvements).

[PaulOlteanu]

I have some tests written as well https://github.com/PaulOlteanu/rspotify/commit/2d38341478bfa950225a2fbd8e6e5d6ddcb4c5ac if you'd like to add those

[ramsayleung]

Thanks for your contribution, I would like to add your test, but I refine the callback function based your code.

The callback function is a field of Config struct instead of SpotifyClient, so it's unnecessary to use Arc<Mutex> to sync between different threads.

TokenCallback should only be invoked when a token is gained successfully, so it's unnecessary to use Fn(Option<Token>, Fn(Token) is more appropriate.

Furthermore, the callback function might throw an error, so the return value of the callback function should be Result

[PaulOlteanu]

Yeah I had them written up for my original implementation - I just added them in case you wanted to modify them to fit this pr

[ramsayleung]

Sure, thanks for your contribution in advance.