feat: add support for OAuth clientCredential and password flows in Respect core

.changeset/oauth2-x-security-token-exchange.md

@@ -0,0 +1,7 @@
+---
+'@redocly/respect-core': minor
+'@redocly/openapi-core': minor
+'@redocly/cli': minor
+---
+
+Added OAuth2 token exchange for `x-security` schemes with the `password` and `clientCredentials` flows. Respect fetches the access token from `tokenUrl` and apply `Authorization: Bearer` to the request, which allows to manually obtain a `accessToken`. The `x-security-scheme-required-values` rule now validates the credentials required by the declared flow. Pre-fetched `accessToken` values continue to work.