Eth2Near crates: use `cargo` workspaces for package management

As we have multiple crates in `eth2near` directory, and they somehow depend on each other (on utilization), it makes a lot of sense to manage all of them as a single workspace. It makes sense to utilize [cargo workspaces](https://doc.rust-lang.org/cargo/reference/workspaces.html#the-package-table) to manage the common fields of the package: authors, rust-version, license, etc.