Customizable token cache

[rayluo]

Inspired by its cousin in MSAL .Net, this PR refactors MSAL Python's token cache to provide an INTERNAL helper which can store additional values from token request or response.

Implementation details (which are not exactly the same as MSAL .Net, but the overall purpose is the same):

• We have to differentiate between request and response because they are two different data sources in this code base. And the use case (CDT experiment) needs both of them.
• The same additional values are also used in token search.
• Currently, the changes above are only applicable to access token.
• The preexisting hard-coded key_id was also refactored this time to utilize the new generic declaration, and the existing test case was updated to reflect that.
• This PR does not bring other behavior changes, and all the existing test cases still pass. So, it should be safe to merge.
• Some other prototypes for recent projects will be built on top of this PR.