Replace isinstance call in Pipeline with structural checks

[chlowell]

Today, Pipeline calls isinstance(policy, SansIOHTTPPolicy) to decide whether to wrap a policy with a runner defining send(). Explicit type checking is contrary to our guidelines' preference for structural subtyping and it may also be useful to allow a SansIOHTTPPolicy to define its own send(). So, this PR replaces the isinstance call with structural checks: if a policy implements send(), Pipeline calls that method; otherwise, if a policy implements SansIOHTTPPolicy's methods, Pipeline wraps that policy with a runner. If an alleged policy doesn't define any of these methods, e.g. it's None, Pipeline ignores it.

[lmazuel]

Guess this deserves a else now that raises that it won't work. Before, we were appended the policy and praying and it would have failed at the first "pipeline.run". Now, it silently drops the object passed as policy (silent drops are dangerous)

[chlowell]

Sure, that's reasonable. Breaks a few (hacky) tests for Key Vault (fixed by #16784), we'll see what else. Pipeline still needs to silently ignore None in policy lists because Configuration uses that as a default value for policies; I added test coverage of that.

[xiangyan99]

Should we have this in policies/_base.py to make it live with SansIOHTTPPolicy definition?

[chlowell]

Good idea, have done.

[chlowell]

Closing this because it's no longer blocking anything.