feat: add pkce support

[bshaffer]

In order to support PKCE for OAuth2, adds the following public methods:

• OAuth2::generateCodeVerifier(): string
• OAuth2::setCodeVerifier(?string $codeVerifier): void
• OAuth2::getCodeVerifier(): ?string

Usage

Authorization Request

$auth = new OAuth2(['client_id' => $clientId, 'client_secret' => $clientSecret]);

// Example of how a developer might save the code verifier in a stateless application
// NOTE: calling "generateCodeVerifier" will enable PKCE
$_SESSION['code_verifier'] = $auth->generateCodeVerifier();

// The authorization URI will include "code_challenge" and "code_challenge_method"
// querystring parameters if a code verifier is supplied
// (e.g. https://example.com?code_challenge=[base64 code_verifier]&code_challenge_method=S256)
redirect($auth->buildFullAuthorizationUri());

Token Request

$auth = new OAuth2(['client_id' => $clientId, 'client_secret' => $clientSecret]);

// example of how a developer might retrieve the code verifier in a stateless application
$codeVerifier = $_SESSION['code_verifier'];

// The token URI will include "code_verifier" in the request body if a code verifier is supplied
$auth->setCodeVerifier($codeVerifier);
$token = $auth->fetchAuthToken();

See b/270198724

[TimurSadykov]

LGTM, please let Carl to look as well

[clundin25]

Awesome stuff!

[vishwarajanand]

Lgtm