`SecurityContext#delegated_creds` should be a property with documentation

[DirectXMan12]

Currently, SecurityContext#delegated_creds is just a normal instance attribute of SecurityContext, which means that it can be accidentally overwritten, and it does not show up in the documentation. This should be fixed.

[simo5]

I think it should be possible to drop the delegated credentials esp if we allow to pickle the whole security context and then restore it.
Someone may want to pass around a security context but not transfer any delegated credentials to a lower priviliged process as dellegated credentials may include a delegated tgt.

[DirectXMan12]

Pickling the SecurityContext uses a special pickle process that doesn't actually pickle the object -- it just uses export_sec_context() (so the unpickling process just reconstitutes the SecurityContext using import_sec_context()).

[simo5]

This means delegated credentials get lost in the process.
On the one hand I think this is more secure, OTOH it may come as a suprise to the user and should be carefully documented.

[DirectXMan12]

Yeah. The only reason delegated creds are stored as a field is so that the "step" method can return only a token. It should definitely be documented, though