Add workspace-enabled example contracts (was PR:#44)

[tenuki]

This PR creates a workspace where dependencies are defined and several smart-contracts as part of the same workspace, with inherited dependencies.

We tried different strategies on how to run the workspace contracts' build, and we chosen to try that build as a "whole" analogous to a contract in the test matrix. The execution will be then cartesian product with the platforms.

Having the whole workspace-contracts in that way, we expect it to be all contracts in the workspace compiled at once.

We also included the suggestions given in the PR:44 before.

Was:

At the moment there are a few errors on cargo-contract preventing us to run it completely flawlessly, mainly : 
 * https://github.com/paritytech/cargo-contract/issues/1172
 * https://github.com/paritytech/cargo-contract/issues/1265
That is why we chosen to run the suggested build with `cargo build` instead. But left commented out what 
would be expected to be working in the future when above tickets get fixed.

Updated to:
Updated the CI/CD to what it should be: cargo contract build ... which will fail right now, but have no problems when the cargo-contract version is v 4.0 including: use-ink/cargo-contract#1358 .

[tenuki]

Checked the actual code in the workspace examples and have no problems when ran with cargo-contract v 4.0.