Mutual TLS (mTLS) handling in fetch() and Request

[panva]

Original issue: WinterTC55/fetch#3

I would like to propose that the TC pursue standardization of Mutual TLS authentication extensions to the fetch() and Request API to accomodate both making API requests that employ Mutual TLS authentication, as well as being able to access client certificate information as part of incoming Request objects. These are quite possibly two different features.

[lucacasonato]

Yeah, that seems very worthwhile - probably this is in the context of a larger discussion around "clients" / "agents", which would allow configuration of further options such as:

• h1 vs h2 (and forced h2)
• connection pooling
• proxy
• custom ca cert
• mtls

[lucacasonato]

Next steps:

• What features do different runtimes actually expose?
• How could this look as an extension to fetch

[panva]

What features do different runtimes actually expose?

I will prep this in the coming weeks when I have the capacity.