feat: implement Fetch function and FetchResponse type for HTTP requests

[pikann]

This pull request introduces a new outbound HTTP request capability for WASM plugins by adding a Fetch API, along with the supporting types and host function bindings. The changes are organized into two main themes: the implementation of the Fetch API for WASM builds, and the addition of stubs for non-WASM builds to maintain compatibility.

New Fetch API for WASM plugins:

• Added a FetchResponse struct and a Fetch function to wasm_backends.go, enabling outbound HTTP requests from WASM plugins via the paca.fetch host function. The implementation includes request serialization, host call invocation, and response decoding with error handling.
• Declared the hostFetch function in wasm_imports.go to bind the WASM import for HTTP requests, allowing WASM code to call out to the host for HTTP operations.
• Imported the fmt package in wasm_backends.go to support error formatting in the new Fetch implementation.

Non-WASM compatibility:

• Added a stub FetchResponse struct and a Fetch function in native_backends.go that always returns an error, ensuring non-WASM builds compile and fail gracefully if Fetch is called.

[Copilot]

Pull request overview

This PR adds an outbound HTTP request capability for WASM plugins by introducing a new Fetch API and FetchResponse type, plus the corresponding WASM host import binding, while keeping non-WASM builds compiling via stub implementations.

Changes:

• Added hostFetch WASM import binding for the paca.fetch host function.
• Implemented Fetch and FetchResponse for wasip1 builds with JSON request/response marshalling.
• Added non-WASM stubs for Fetch / FetchResponse that return errNotWASM.

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated 1 comment.

File	Description
wasm_imports.go	Adds the paca.fetch host import binding (hostFetch).
wasm_backends.go	Implements Fetch + FetchResponse for WASM, including host-call wiring and response decoding.
native_backends.go	Adds non-WASM stubs for Fetch and FetchResponse to preserve build compatibility.

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