Switch to Actix (awc)

[trueb2]

Switch to using a recent version of awc on an actix_rt

Realistically, I plan on using this with an actix deployment, there is no reason to use hyper or reqwest.

One of the focuses of #24 was compile time, a cargo clean && cargo build --release --example simple_sender took

   Compiling awc v3.0.0-beta.18
   Compiling fcm v0.9.1 (/Users/jwtrueb/Desktop/workspace/fcm-rust)
    Finished release [optimized] target(s) in 38.60s

and a cargo clean && cargo build --release --example simple_sender --features openssl

   Compiling awc v3.0.0-beta.18
   Compiling fcm v0.9.1 (/Users/jwtrueb/Desktop/workspace/fcm-rust)
    Finished release [optimized] target(s) in 41.38s

Here are the some of the features available on awc

[package.metadata.docs.rs]
# features that docs.rs will build with
features = ["openssl", "rustls", "compress-brotli", "compress-gzip", "compress-zstd", "cookies"]

[features]
default = ["compress-brotli", "compress-gzip", "compress-zstd", "cookies"]

# openssl
openssl = ["tls-openssl", "actix-tls/openssl"]

# rustls
rustls = ["tls-rustls", "actix-tls/rustls"]

I set the default feature rustls, since we need SSL

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This change is 

[trueb2]

You might want to break out the features and choose reqwest, hyper, awc, and tls based on features instead of using this PR.