Implemented workspace support

[rasmus-kirk]

Purpose

We want cargo-concordium to support workspaces.

• Support building a crate in a workspace by navigating into the subcrate directory and running cargo-concordium build
• Support reproducible builds in workspaces.
• Support specifying profiles with a --profile flag
• Changelog, documentation...

There's still two issues remaining:

• Running cargo concordium test would fail when run in the workspace root, as the contract integration tests need to build the WASM first. I suggest refactoring and exposing the build function in this crate and then building the contract in the integration tests themselves, with the added bonus of making the integration tests part of rust's own testing ecosystem.
• The reproducible builds listen to the rust-toolchain.toml file. Ideally, we would want it to ignore that and always use the provided configuration set by the docker environment. I don't know how to address this. There's been an open issue for the docker image without feedback for more than a year 💀 (I think we can maybe not include it in the archive, I'll test more)

Checklist

• My code follows the style of this project.
• The code compiles without warnings.
• I have performed a self-review of the changes.
• I have documented my code, in particular the intent of the
  hard-to-understand areas.
• (If necessary) I have updated the CHANGELOG.

[limemloh]

Please check that you can still verify a verifiable build from an older version of cargo-concordium with these changes.

[limemloh]

Please add more detail to this, support for workspace is not saying much. Instead, describe what is now possible compared to before.