Support compiling to wasm32 architectures

[lulf]

Summary

Enable support for compiling sigstore-rs to WASM, making it usable from browser applications and on wasm32 VMs.

Example application using the Yew framework: https://github.com/trustification/sigstore-search

Closes #220

[flavio]

That's cool. Would it be possible to add an example of this kind of usage?

[flavio]

I'm not sure this is something we have to do, quoting the documentation of the getrandom crate:

This feature should only be enabled for binary, test, or benchmark crates. Library crates should generally not enable this feature, leaving such a decision to users of their library. Also, libraries should not introduce their own js features just to enable getrandom’s js feature.

I think a better approach would be to:

• Extend our documentation explaining how to use this crate when one of the WebAssembly targets is used
• Move the getrandom dependency to the dev-dependencies. This will ensure the unit tests can be run

[lulf]

Would it make sense to add a 'wasm' feature (and document it) to the crate that enabled it instead? I think it's a bit strange to require 'user' crates to set features on transient dependencies in order for things to compile.

[flavio]

I've already ran into some crates that require that, so it doesn't feel so strange to me. However, I think adding a wasm feature would be better

[lulf]

I've update to enable the js flag only if the wasm feature is enabled, and also added some info to the readme.

[flavio]

LGTM, can be merged once all the tests are green

[lulf]

Clippy and integration tests compile now, and I squashed the commits. I think the GHA should be correct as well.