chore!: ensure consistent secret token generation and hashing (#20388)

This PR uses the same sha256 hashing technique as we use for APIKeys. So
now all randomly generated secrets will be hashed with sha256 for
consistency.

This is a breaking change for the oauth tokens. Since oauth is only
allowed for dev builds and experimental, this is ok.

codersdk/oauth2.go

@@ -486,6 +486,6 @@ type OAuth2ClientConfiguration struct {
 	TokenEndpointAuthMethod string          `json:"token_endpoint_auth_method"`
 	Scope                   string          `json:"scope,omitempty"`
 	Contacts                []string        `json:"contacts,omitempty"`
-	RegistrationAccessToken string          `json:"registration_access_token"`
+	RegistrationAccessToken []byte          `json:"registration_access_token"`
 	RegistrationClientURI   string          `json:"registration_client_uri"`
 }