Add workspace support

[chesedo]

When I deploy a crate that is within a workspace and that has a { workspace = true } dependency, then I get an error that the manifest cannot be parsed.

The reason for this is because shuttle did not archive the entire workspace, but only the single crate.

[gautamprikshit1]

Any help on how to achieve this

[chesedo]

I can think of the following changes that will be needed (each in a separate PR):

1. Our way of finding the Cargo.toml file is wrong since it currently stops at the first one found. Rather it should do whatever cargo is doing to continue looking in the parent tree for another Cargo.toml that might be the workspace one. When shuttle collects the correct Cargo.toml (for the workspace) then I believe everything is already in place for shuttle to correctly archive the entire workspace. Identifying the correct Cargo.toml is a change that needs to happen in cargo-shuttle somewhere.
2. We have a loader.rs file (in service) which builds an archive, but then only gets the first cdylib from the build. Since the archive is now the entire workspace there might be multiple cdylibs so this build method needs to be updated to return a Vec<_>. The caller might then also need a small update, but I believe shuttle has everything else in place to support running multiple services / crates in a workspace.

[brokad]

Just to confirm here @chesedo, I suppose this also requires a change in deployer somewhere since a workspace could contain multiple services, each of which needs recognition in the deployer's state?

[chesedo]

@brokad the deployer state can already start multiple services. For services that depend on one another, starting them in the correct order is just missing.

[chesedo]

Change number 2 for the above comment is implemented in #767