Enable verifiable builds for smart contract projects with local dependencies

[DOBEN]

Task description

Verifiable builds currently only work in the case of self-contained Rust smart contract projects (pulling dependent libraries/crates from e.g. crates.io) but no local dependencies. More complex smart contract protocols (such as the Umbrella oracle protocol) often have local dependencies by referencing contracts in different Rust modules.

Implement a workspace feature for it:
#157 (comment)

Enable verifiable builds for projects:

Sub-tasks

• having local dev-dependencies

e.g. https://github.com/Concordium/concordium-rust-smart-contracts/blob/main/examples/auction/Cargo.toml#L18

• having local dependencies

e.g. https://github.com/Concordium/concordium-rust-smart-contracts/blob/main/examples/auction/Cargo.toml#L15