feat: TLS support for Spin.

[fibonacci1729]

closes #33

The spin-cli now supports the tls-key and tls-cert options, e.g.
spin up --tls-key <path/to/key> --tls-cert <path/to/cert> ...

For convenience, the following environment variables can be set in lieu of
their respective flags.

SPIN_TLS_CERT=<path/to/crt>
SPIN_TLS_KEY=<path/to/key>

NOTE: Explicitly set flags take precedence over the environment.

export RUST_LOG=spin_engine=info,spin_http,wact=info
export SPIN_TLS_CERT=crates/http/tests/local.crt.pem
export SPIN_TLS_KEY=crates/http/tests/local.key.pem
spin up --f templates/spin-http/spin.toml
curl -k https://127.0.0.1:3000/test/hello

Signed-off-by: Brian Hardock brian.hardock@fermyon.com
Co-authored-by: Michelle Dhanani michelle@fermyon.com

[radu-matei]

There is one more thing that should be updated, and that is the header with the full URL of a request.
Currently, the scheme of the URL is always http — see

spin/crates/http/src/lib.rs

Lines 216 to 218 in 6651c2c

	// TODO: check if TLS is enabled and change the scheme to "https".
	let scheme = "http";
	let full_url = format!("{}://{}{}", scheme, host, abs_path);

[radu-matei]

After Clippy is happy, LGTM.

Thanks!