Add support for aws_lc_rs as crypto backend (#201)

Starting from [0.22.0,
`rustls`](https://github.com/rustls/rustls/releases/tag/v%2F0.22.0),
ecosystem has started supporting
[`aws-lc-rs`](https://github.com/aws/aws-lc-rs) as crypto back-end. This
latter replaces crypto functionalities provided by `ring`.

The main reason behind this is to provide
[FIPS](https://en.wikipedia.org/wiki/FIPS_140-2) compliance in the
future via `aws-lc`:

> Optional support for cryptography from
[aws-lc-rs](https://github.com/aws/aws-lc-rs). Once the certification
process completes , we will support FIPS mode using aws-lc-rs.

---

This PR is implementing the
[same](https://github.com/rustls/rustls/blob/4d1b762b5328a1714862ba73ec72d5522fe0c049/rustls/Cargo.toml#L19)
supporting mechanism.

In short:
 * if cfg(feature = "ring") -> ring   (this will be the default)
 * if cfg(feature = "aws-lc-rc") -> aws-lc-rs
 * if both -> ~compile_error!~ EDIT: ring
 * if none -> compile_error!

Author: BiagioFesta

.github/workflows/ci.yml

@@ -39,7 +39,7 @@ jobs:
         with:
           components: clippy
       - run: cargo clippy --all-features --all-targets
-      - run: cargo clippy --no-default-features --all-targets
+      - run: cargo clippy --no-default-features --features aws_lc_rs,pem --all-targets
 
   rustdoc:
     name: Documentation
@@ -117,6 +117,8 @@ jobs:
         arch: amd64
     - name: Install Rust
       uses: dtolnay/rust-toolchain@stable
+    - name: Install NASM for aws-lc-rs on Windows
+      uses: ilammy/setup-nasm@v1
     - run: echo "VCPKG_ROOT=$env:VCPKG_INSTALLATION_ROOT" | Out-File -FilePath $env:GITHUB_ENV -Append
     - run: vcpkg install openssl:x64-windows-static-md
     - name: Run cargo check
@@ -125,8 +127,8 @@ jobs:
       run: cargo test --all-targets
     - name: Run the tests with x509-parser enabled
       run: cargo test --verbose --features x509-parser --all-targets
-    - name: Run the tests with no default features enabled
-      run: cargo test --verbose --no-default-features --all-targets
+    - name: Run the tests with aws_lc_rs backend enabled
+      run: cargo test --verbose --no-default-features --features aws_lc_rs,pem --all-targets
 
   build:
     strategy:
@@ -164,8 +166,8 @@ jobs:
       run: cargo test --all-targets
     - name: Run the tests with x509-parser enabled
       run: cargo test --verbose --features x509-parser --all-targets
-    - name: Run the tests with no default features enabled
-      run: cargo test --verbose --no-default-features --all-targets
+    - name: Run the tests with aws_lc_rs backend enabled
+      run: cargo test --verbose --no-default-features --features aws_lc_rs,pem --all-targets
 
   coverage:
     name: Measure coverage