security/advancedtls: add SkipServerAuthEKU option to bypass server EKU verification (#9222)

This adds `SkipServerAuthEKU` option to `Options`. When set to `true`,
the client will verify server certificates using `x509.ExtKeyUsageAny`
instead of requiring `x509.ExtKeyUsageServerAuth`. This is useful when
the server's certificate does not possess the Server Auth EKU.

BUG=528004991

RELEASE NOTES:
* security/advancedtls: Add `SkipServerAuthEKU` option to skip EKU check
during server authentication.