TLS backend selection feature flags, Default, clippy

[virtualritz]

• Exposed reqwest's TLS backend selection features.
  The main reason is that request's default TLS dependency pulls in openssl which wraps the OpenSSL C-lib via openssl-sys.
  This not only adds a noticable hiccup to build times, it also requires a resp. C compiler toolchain which can be a PITA when cross-compilation for other platform targets is needed (e.g. Android or iOS).

  reqwest offers a bunch of alternative TLS backends, exposed by feature flags.

  With this PR they're all also available in the fcm_v1 crate.

• Made clippy happy:

  • Swapped explicit for derived Default for some types.
  • Removed superfluous return.

• Cleaned up/added docs.

[virtualritz]

Ping!