Require more explicit opt-in to unstable wasip3 executor

[lann]

We'd like to not support the current wasip3 RC indefinitely, especially given that there is some major interface renaming happening soon.

We already have a signal in the Rust SDK with the wasip3-unstable feature flag and I'd like a matching opt-in for Spin manifests:

• Add support for a new http trigger executor type:

  [[trigger.http]]
  executor = { type = "wasip3-rc-2025-09-16" }
  ...

• If the component exports the p3 interface but this executor type isn't set, fail with a helpful error, like:

  Support for wasi:http 0.3.0-rc-2025-09-16 is experimental and will be removed in a future Spin release.
  You can opt-in to this unstable interface by adding executor = { type = "wasip3-rc-2025-09-16" } to
  the appropriate [[trigger.http]] section of your spin.toml file.