* Debugging: add builtin gdbstub component.

This adds a debug component that makes use of the debug-main world
defined in bytecodealliance#12756 and serves the gdbstub protocol, with Wasm
extensions, compatible with LLDB.

This component is built and included inside the Wasmtime binary, and
is loaded using the lower-level `-D debugger=...` debug-main option;
the user doesn't need to specify the `.wasm` adapter
component. Instead, the user simply runs `wasmtime run -g <PORT>
program.wasm ...` and Wasmtime will load and prepare to run
`program.wasm` as the debuggee, waiting for a gdbstub connection on
the given TCP port before continuing.

The workflow is:

```
$ wasmtime run -g 1234 program.wasm
[ wasmtime starts and waits for connection ]

$ /opt/wasi-sdk/bin/lldb  # use LLDB from wasi-sdk release 32 or later
(lldb) process connect --plugin wasm connect://localhost:1234
Process 1 stopped
* thread #1, stop reason = signal SIGTRAP
    frame #0: 0x40000000000001cc
->  0x40000000000001cc: unreachable
    0x40000000000001cd: end
    0x40000000000001ce: local.get 0
    0x40000000000001d0: call   13
(lldb) si
Process 1 stopped
* thread #1, stop reason = instruction step into
    frame #0: 0x4000000000000184
->  0x4000000000000184: block
    0x4000000000000186: block
    0x4000000000000188: global.get 1
    0x400000000000018e: i32.const 3664
[ ... ]
```

This makes use of the `gdbstub` third-party crate, into which I've
upstreamed support for the Wasm extensions in daniel5151/gdbstub#188,
daniel5151/gdbstub#189, daniel5151/gdbstub#190, and
daniel5151/gdbstub#192. (I'll add vets as part of this PR.)

* cargo vets.

* Handle Trap events as well as breakpoints.

* Review feedback.

* Fix gdbstub artifact build to make it publishable (by disabling it when isolated crates are used).

* Review feedback.

* fix published-crates list

* For now, empty gdbstub data but no compile error when artifact crate is published.

* add some more Cargo metadata: version for artifact crate dep