Stop forcing rustls in the `future_snark` feature of STM

[jpraynaud]

Why

The future_snark feature unconditionally enables rustls, with a comment stating it is temporary. The crate also exposes a native-tls feature and documents that exactly one TLS backend must be selected, so native-tls is currently unselectable and the choice is taken away from the caller.

What

Let the TLS backend of the SRS download be selected by the downstream crate rather than forced by future_snark.

How

• Remove rustls from the future_snark feature list (mithril-stm/Cargo.toml)
• Select the TLS backend explicitly in the crates that enable future_snark
• Fail the build with a clear message when no TLS backend or more than one is enabled
• Check that the WASM target is unaffected