Add a PolkaVM-based executor (#3458)

This PR adds a new PolkaVM-based executor to Substrate.

- The executor can now be used to actually run a PolkaVM-based runtime,
and successfully produces blocks.
- The executor is always compiled-in, but is disabled by default.
- The `SUBSTRATE_ENABLE_POLKAVM` environment variable must be set to `1`
to enable the executor, in which case the node will accept both WASM and
PolkaVM program blobs (otherwise it'll default to WASM-only). This is
deliberately undocumented and not explicitly exposed anywhere (e.g. in
the command line arguments, or in the API) to disincentivize anyone from
enabling it in production. If/when we'll move this into production usage
I'll remove the environment variable and do it "properly".
- I did not use our legacy runtime allocator for the PolkaVM executor,
so currently every allocation inside of the runtime will leak guest
memory until that particular instance is destroyed. The idea here is
that I will work on the https://github.com/polkadot-fellows/RFCs/pull/4
which will remove the need for the legacy allocator under WASM, and that
will also allow us to use a proper non-leaking allocator under PolkaVM.
- I also did some minor cleanups of the WASM executor and deleted some
dead code.

No prdocs included since this is not intended to be an end-user feature,
but an unofficial experiment, and shouldn't affect any current
production user. Once this is production-ready a full Polkadot
Fellowship RFC will be necessary anyway.