[Kestrel] Certificate chains support

[javiercn]

Right now Kestrel supports loading certificates from PFX files and PEM files, but it only loads one certificate.

• That forces the certificate chains to be resolved within the SSL stream which require going to the network to resolve the chain.
• There are new APIs on SslStream to provide the chain as well as the certificate to use that avoid having to fetch the chain from the network.
• We want to load entire certificate chains from PFX and PEM certificate files to pass them directly to SslStream and avoid having to resolve the chain.

[Pilchie]

Is this the same thing as #21513?

[javiercn]

@Pilchie I think so, or it overlaps a bit. David is taking care of it.

[Pilchie]

Okay, moving to 5.0.0-rc2 then.