Add support for PKCE for OAuth2

[thekid]

Example:

use web\auth\oauth\{OAuth2Flow, ByPKCE};

$flow= new OAuth2Flow(
  'https://example.com/login/oauth/authorize',
  'https://example.com/login/oauth/access_token',
  new ByPKCE($clientId, 'S256'),
  '/oauth/callback'
);

Implements client side of xp-forge/mcp#11

[thekid]

Released in https://github.com/xp-forge/web-auth/releases/tag/v7.0.0