OAuth 2.0 PKCE challenge (RFC7636)

[byewokko]

closes #139

PKCE is always enabled in Seacat Auth; it is not necessary to configure the app.

Both plain and S256 code challenge methods are supported.

To be able to use PKCE with your client:

• Register your client and set its code_challenge_methods attribute to ["S256"] (or ["plain"] if SHA256 is not implemented on the client server)
• A client is allowed to register only one method at once, i.e. it is not possible to set {"code_challenge_methods": ["S256", "plain"]}

The PKCE code challenge works as described in RFC7636

[ateska]

Please also include PKCE in the README.md as a supported feature - including link to the PKCE specifications.

[ateska]

https://github.com/TeskaLabs/seacat-auth/blob/main/README.md updated also.