[deno] Associate external memory with each command encoder on Metal

[andyleiserson]

This encourages doing garbage collection before the command buffer limit is reached.

This change is complementary to #8574. That change will change deadlocks into device loss, in any environment where wgpu runs. This change avoids reaching the limit, only in Deno.

I made the change specific to Metal, and not specific to cts_runner. I don't have strong feelings about either of those things, i.e., I'd also be fine doing it on all platforms, and I'd be fine adding a cts_runner feature so that we can do this only in cts_runner and not in other contexts where Deno runs.

Connections
Related to #3084.

Testing
Tested locally that this resolves some CTS suites that were hanging. However, both suites still have failing tests, so can't be run as a single selector in CI.

I also got a hang in CI on #8699 that I suspect is this issue, I will confirm that #8699 can run the CTS reliably with this fix.

Squash or Rebase? Squash

Checklist

• Run cargo fmt.
• Run taplo format.
• Run cargo clippy --tests. If applicable, add:
  • --target wasm32-unknown-unknown
• Run cargo xtask test to run tests.
• If this contains user-facing changes, add a CHANGELOG.md entry.

[ErichDonGubler]

I think this is fine? Let's merge it, and have @crowlKats tell us if it's wrong when he's got the time.