Allow passing custom crypto provider

See [comment here](https://github.com/bitcoindevkit/rust-electrum-client/pull/135#issuecomment-2244180324), and idea for how to tackle [here](https://github.com/bitcoindevkit/rust-electrum-client/pull/135#issuecomment-2253683130).

> 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.