initial idea for clarifying names of token -> token_dict and token_str

[jvanasco]

Here is an initial idea to start addressing #856

Various functions accept a parameter with a name like "token", but the parameter might either be a String or a Dict depending on the function. Sometimes two similar functions will require different input types - for example save_bearer_token operates on a Dict, but validate_bearer_token operates on a string. To better clarify this, and aid in future typing, the names of these functions were updated to have a _str or _dict suffix. On several functions, the type of input was not identified in the docstring and a code audit was needed to determine.

This does not change the various attributes on oauthlib.common.Request - which do need to be better listed and typed.

This quick PR was done for initial feedback. I did not run this through any tests yet.

[auvipy]

the name change scheme seems to be OK but need input from more maintainers. also the related tests also should be renamed I think