Add PKCE and update VK and Yandex flow

[LacticWhale]

Hey!

Type: new feature

Link to issue:
#171

In raising this pull request, I confirm the following (please check boxes):

• I have read and understood the Contributing Guidelines.
• I have checked that another pull request for this purpose does not exist.
• I wrote some tests for this PR.

Small description of change:

• Add PKCE (required for oauth2.1)
• Update vk to oauth2.1
• Update yandex

Thanks 😺