ci: add separate license check

[jmayclin]

Description of changes:

Ensure that the license and the copyright is always included.

Testing:

I tested this by reducing the number of lines that it checked for and confirming that error were emitted. For example, if you only use head -4 instead of head -5, the following errors are printed out

License Check Failed: /home/ec2-user/workspace/s2n-tls/bindings/rust/s2n-tls-sys/lib/utils/s2n_safety_macros.h
License Check Failed: /home/ec2-user/workspace/s2n-tls/bindings/rust/s2n-tls-sys/lib/utils/s2n_rfc5952.h
License Check Failed: /home/ec2-user/workspace/s2n-tls/utils/s2n_rfc5952.h
License Check Failed: /home/ec2-user/workspace/s2n-tls/utils/s2n_safety_macros.h
License Check Failed: /home/ec2-user/workspace/s2n-tls/tests/fuzz/s2n_extensions_client_supported_versions_recv_test.c
License Check Failed: /home/ec2-user/workspace/s2n-tls/codebuild/bin/install_apache2.sh
License Check Failed: /home/ec2-user/workspace/s2n-tls/codebuild/bin/install_gnutls37.sh
License Check Failed: /home/ec2-user/workspace/s2n-tls/codebuild/bin/install_shellcheck.sh
License Check Failed: /home/ec2-user/workspace/s2n-tls/bindings/rust/s2n-tls-sys/src/features/internal.rs
License Check Failed: /home/ec2-user/workspace/s2n-tls/bindings/rust/s2n-tls-sys/src/features/quic.rs
License Check Failed: /home/ec2-user/workspace/s2n-tls/bindings/rust/s2n-tls-sys/src/features/crl.rs
License Check Failed: /home/ec2-user/workspace/s2n-tls/bindings/rust/s2n-tls-sys/src/features/fingerprint.rs
License Check Failed: /home/ec2-user/workspace/s2n-tls/bindings/rust/s2n-tls-sys/src/features/npn.rs
License Check Failed: /home/ec2-user/workspace/s2n-tls/bindings/rust/s2n-tls-sys/src/features/ktls.rs
License Check Failed: /home/ec2-user/workspace/s2n-tls/bindings/rust/s2n-tls-sys/src/features/renegotiate.rs
License Check Failed: /home/ec2-user/workspace/s2n-tls/bindings/rust/s2n-tls-sys/src/api.rs

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.