Implement OAuth 2.0 authentication

[paulius-valiunas]

Tell us about the task you want to perform and are unable to do so because the feature is not available

I want to send requests to services that use OAuth 2.0. Right now I have to manually acquire an authorization token and copy/paste it into APIDash. The tokens expire usually in an hour, so I have to do this multiple times a day, not only separately for every service, but even for every individual request I have in the library.

Describe the solution/feature you'd like us to add

APIDash should automatically open a browser session/popup and let me sign in using OAuth 2.0's "hybrid", "implicit" flows. It should also allow me to obtain a token using the "client credentials" flow, which doesn't require a browser session (this is way easier to implement and would already be a huge step forward).

[ashitaprasad]

Thanks for opening this issue @paulius-valiunas
Tagged it as a good first issue so that contributors can also take a look into it and send across a PR