Switch to `bitreq`

[TheBlueMatt]

Now that we finally have a minimal-dependency async HTTP client in the form of bitreq - https://github.com/rust-bitcoin/corepc/tree/master/bitreq we should try to start switching crates over to it.

[tnull]

Yeah, we should eventually, though of course we'll to check that that bitreq already supports all features we need in vss-client-ng and LDK Node first.

[tnull]

FTR, we'll need them to resolve at the very least rust-bitcoin/corepc#441 and rust-bitcoin/corepc#442 before we can move forward with it for VSS.