Make Reqwest TLS backend configurable (native-tls/rustls-tls)

[rb090]

This PR refactors how bbjwt depends on reqwest by disabling its default features and instead features are exposed to let users choose the TLS backend.

By default, native-tls (OpenSSL) remains enabled, but consumers can now opt into rustls instead:

# Example: switch to Rustls in downstream crate
bbjwt = { version = "0.x", default-features = false, features = ["tls-rustls"] }