feat(h3): allow multi impl

[FrankSpitulski]

Relates to #1558

other impls of hyperium/h3 can implement the H3ConnectionProvider trait allowing use of more than one h3 impl and also further customization of the default quinn transport.

I couldn't work out how to let the user provide a dyn trait without exposing the generic h3 types everywhere. maybe someone who knows rust a bit better can help. the trait should be moved to a user facing package too, but I'd like to get the impl sorted first.

[seanmonstar]

Thanks for thinking about this! I feel this is similar to the conversation in #1786. In short, I worry about exposing too much of the internals, because then we can't change things around.

[FrankSpitulski]

my goal with this was to allow users to provide “something that implements h3”, which I think is still in the spirit of not too much internals. I just couldn’t get the trait to work without infecting the rest of the library with generics for the h3 concrete type, despite the dyn trait box.

h3 is still part of hyper, and imo sufficiently generalised. I wouldn’t want to directly expose quinn types though.

[FrankSpitulski]

from what I can tell because the h3 traits have associated types, they are not object safe and cannot be referenced without knowing specifying those impl types.

https://stackoverflow.com/questions/67213532/vector-of-polymorphic-structs-with-an-associated-type-in-trait