Introduce `runtime-fuzzer` with `wasmer` backend

[techraed]

That could be reached with implementing such a fuzz target, that constructs a node client pretty similar to https://github.com/gear-tech/gear/blob/983e979928b0ae460794c09423d21c8e4bf6c54b/node/service/src/client.rs and fuzzes node with extrinsics sent by calling one of the node services. This will give the fuzz test of the very realistic client+runtime (very little mocks), tests will be on the real storage and, as a result, wasmer backend will be fuzzed.

to be done after #2187