feat: digest is configurable

[DXist]

This PR allows to selectively configure supported content digest crypto hash functions using cargo features.

When neither "digest-sha256" nor "digest-sha512" feature is enabled, the content digest code is completely configured out.

[rinatshigapov@MacBook-Pro-Rinat httpsig-rs (feat/configurable_content_digest)]$ cargo build --no-default-features --features="blocking digest-sha512"
    Finished `dev` profile [unoptimized + debuginfo] target(s) in 0.20s
[rinatshigapov@MacBook-Pro-Rinat httpsig-rs (feat/configurable_content_digest)]$ cargo build --no-default-features --features="blocking digest-sha256"
    Finished `dev` profile [unoptimized + debuginfo] target(s) in 0.03s
[rinatshigapov@MacBook-Pro-Rinat httpsig-rs (feat/configurable_content_digest)]$ cargo build --no-default-features --features="blocking"
    Finished `dev` profile [unoptimized + debuginfo] target(s) in 0.03s

[DXist]

I think digest features should be opt-in by default, because there might be non-sensitive use cases, where it's enough to sign only request metadata without a content payload.

Also, content digest is not trivial for streaming requests.