Add WASI 0.3.0 support

[dicej]

WASI 0.3.0 was finalized today, and Wasmtime 46 will support it by default. So should Spin.

Note that, just as with 0.2.0, we'll continue supporting the 0.3.0-rc-2026-03-15 snapshot indefinitely as well.

[cschanhniem]

When upgrading from the 0.3.0-rc-2026-03-15 snapshot to WASI 0.3.0 final, the main breaking change to watch for is in wasi:http/types: the fields resource was renamed to header-fields and FieldName, FieldKey, FieldValue types were reorganized under a header-types interface. The outbound HTTP codepath in crates/outbound-http/src/lib.rs directly constructs these, so that's the primary touchpoint.

The other significant delta is that the wasi:cli/exit interface in 0.3.0 final uses result<_, ()> instead of result<_, _> — the rc snapshot had a second error variant that was removed. Spin's crates/runtime/src/exit.rs (or wherever the CLI exit host implementation lives) will need the function signature updated to match.

Spin's WIT deps currently pin wasi:http/types@0.3.0-rc-2026-03-15 across several world files; a bulk search-and-switch across wit/deps/ plus updating the Wasmtime adapter in crates/wasi-http-adapter/ should cover it. The Wasmtime 46 release notes will have the exact .wit deltas to diff against.

This is also a good opportunity to consider whether Spin wants to pin to the 0.3.0 final snapshot or continue being flexible with snapshot versions — the rc snapshot was the only 0.3.x version with significant production deployment, and breaking it quietly would strand WASI 0.3.0-rc modules that users have already built against Spin's WIT worlds.

[dicej]

This is also a good opportunity to consider whether Spin wants to pin to the 0.3.0 final snapshot or continue being flexible with snapshot versions — the rc snapshot was the only 0.3.x version with significant production deployment, and breaking it quietly would strand WASI 0.3.0-rc modules that users have already built against Spin's WIT worlds.

Hence this part of the issue description:

Note that, just as with 0.2.0, we'll continue supporting the 0.3.0-rc-2026-03-15 snapshot indefinitely as well.