Fix Wasmtime usage

[MaksymZavershynskyi]

It seems like #2853 has changed between the final approval and the merge. We typically do not enforce undoing of the approvals when new commits are pushed after the approvals, because it makes our development cycles slower.

We need to return the following important code into near-vm-runner according to the discussion and the approvals:

• The choice between wasmtime and wasmer should be a compilation flag, which it is currently not: https://github.com/nearprotocol/nearcore/blob/master/runtime/near-vm-logic/src/config.rs#L17

Besides @SkidanovAlex has reported problems with compiling wasmtime on our CI, therefore the compilation flag should make wasmtime dependency optional and only included when the compilation flag is specified, see for the reference https://doc.rust-lang.org/cargo/reference/index.html