Allow passing custom crypto provider

[thunderbiscuit]

See comment here, and idea for how to tackle here.

Can we just expose the rustls provider API here: https://docs.rs/rustls/latest/rustls/crypto/struct.CryptoProvider.html so I can pass in my own provider? We can have a rustls-aws-lc feature that adds a constructor that uses that. Can do the same for rustls-ring. But I'd like to able to fix this kind of issue in the future without having to wait for a merge.

[tnull]

I'd be +1 here. A user of ours just ran into a panic as we used rust-electrum-client with use-rustls, assuming that it would be the counterpart to use-rustls-ring, i.e., be safe to use. Turns out that assumption is wrong as we'd just panic if we don't manually call install_crypto_provider. Now opened #171 to fix this omission.