Support multiple contracts in cargo workspace

[nuke-web3]

Is your feature request related to a problem? Please describe.

I want to use a single repository to house a set of independent contracts, and from the TLD I want to use cargo contract ... to execute on one or more of these contracts.

See:

• ERROR: Cannot infer the root project id inkdevhub/marketplace#3 - similar structure to want I want as a repo with the same ERROR for same reason
• Instantiating contract without manifest-path fails #516 - similar error and suggestion to manually specify the path in a rather verbose way.

Sadly as I must run cargo for each contract in the worspace, I have to rebuild the full target for each member. I want to share a target for this workspace, and a single lockfile for the whole project that I cannot do with the present construction without the manual --manifest-path that isn't as clean to use.

Describe the solution you'd like

With a workspace, cargo contract should iterate through each member and run on each in succession, by default, with perhaps a hint to use --manifest-path if you only want to run on one contract.

Alternatively detect the valid contract manifests and one of:

1. allow me to select in a check box minimal TUI interface the ones I would like to run
2. print the ones detected and the command to use to invoke the command for them (cargo contract ... --manifest-path ./...)

Describe alternatives you've considered

As #516 (comment) suggests, an error that is more helpful than ERROR: Cannot infer the root project id that is what happens now would be good enough, suggesting to use --manifest-path at least.

[ascjones]

See also #182