Add PKCE parameters to relevant OAuth methods

[bocops]

Description

This adds parameters necessary for PKCE (Proof Key for Code Exchange) during OAuth authorization flow (see https://docs.joinmastodon.org/spec/oauth/#pkce). Specifically:

• adds parameters state, codeChallenge, codeChallengeMethod to getOAuthUrl()
• adds parameter codeVerifier to getUserAccessTokenWithAuthorizationCodeGrant()
• same for Rx methods
• modifies Kotlin+Java samples to make use of these parameters

Closes #531.

Type of Change

• New feature

Breaking Changes

• Changed parameter order in getUserAccessTokenWithAuthorizationCodeGrant(). Java users of the library will need to add a null parameter (as codeVerifier) before a non-null scope parameter if not using PKCE.

How Has This Been Tested?

See below. Also ran modified samples against live Mastodon servers to confirm that PKCE parameters are used and correct.

Mandatory Checklist

• I ran gradle check and there were no errors reported
• I have performed a self-review of my code
• I have added tests that prove my fix is effective or that my feature works
• All tests pass locally with my changes
• I have added KDoc documentation to all public methods

Unit tests not possible, but samples do work.

Optional checks

• In case you worked on a new feature: Did you also implement the reactive endpoint (bigbone-rx)?
• Did you also update the documentation in the /docs folder (e.g. API Coverage page)?

Documentation was not yet changed back from "fully supported".

[bocops]

Note regarding the changed parameter order mentioned as a breaking change: I could have added the new parameter to the end, but expect PKCE to eventually become mandatory (or at least best practice), so that a non-null codeVerifier will be more common than a non-null scope.

[codecov]

Codecov Report

Attention: Patch coverage is 31.81818% with 15 lines in your changes missing coverage. Please review.

Project coverage is 48.30%. Comparing base (8b34209) to head (44160b1).
Report is 79 commits behind head on master.

Files with missing lines	Patch %	Lines
...rc/main/kotlin/social/bigbone/rx/RxOAuthMethods.kt	0.00%	8 Missing ⚠️
...n/kotlin/social/bigbone/api/method/OAuthMethods.kt	50.00%	0 Missing and 7 partials ⚠️

Additional details and impacted files

@@             Coverage Diff              @@
##             master     #536      +/-   ##
============================================
- Coverage     49.99%   48.30%   -1.69%     
- Complexity      774      819      +45     
============================================
  Files           176      183       +7     
  Lines          5107     5370     +263     
  Branches        327      289      -38     
============================================
+ Hits           2553     2594      +41     
- Misses         2275     2600     +325     
+ Partials        279      176     -103     

Files with missing lines	Coverage Δ
...n/kotlin/social/bigbone/api/method/OAuthMethods.kt	86.36% <50.00%> (-8.38%)	⬇️
...rc/main/kotlin/social/bigbone/rx/RxOAuthMethods.kt	0.00% <0.00%> (ø)

... and 142 files with indirect coverage changes

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