[corehttp] corehttp SansIOHTTPPolicy is not compatible with existing credentials that use azure.core SansIOHTTPPolicy

[swathipil]

The corehttp Pipeline should accept currently existing Azure credential policies (and others). However, many of these inherit from azure.core SansIOHTTPPolicy. As a result, the policy isn't appended to the SansIOHTTPPolicyRunner b/c of this isinstance check which checks against the corehttp SansIOHTTPPolicy. If we instead make corehttp SansIOHTTPPolicy a runtime_checkable Protocol, the existing credentials should work.