Docs for HTTPS scenarios

[Tratcher]

I'm aggregating some notes here from a variety of issues, questions, discussions, etc. around the HTTPS/TLS capabilities of YARP. These should eventually be written up into a doc.

General:

• Inbound and outbound TLS are independent.
• TLS offloading = inbound HTTPS and outbound HTTP
• No tunneling support

Inbound

• server specific, link to docs for each, summarize capabilities, more details for kestrel as it has more extensibility
• SNI
• hot cert swapping (via SNI callback)
• TLS versions
• perfect score TLS config
• let's encrypt
• log/filter protocols after handshake
• advanced protocol filters before handshake

Outbound

• HTTPS enabled by specifying it in the destination server address
• uses the destination address host name by default. You'll get a TLS error if it's expecting the original host
• There's a transform rule to use the original host, or header transform to use an arbitrary host.
• needs valid certs
• customize HttpClient settings (not yet, Configuring the HttpClientHandler (& Client Certs) #137)

[Tratcher]

Some content: #384 (comment)

[Tratcher]

#1341 added the initial outline for this doc. Do we want to add kestrel specific configuration examples or is the link to kestrel docs sufficient?