Try to load manifest during spin build

[itowlson]

This partly, but only partly, addresses #2520.

With this change, spin build attempts to load the full manifest, falling back to the subset only if that fails, and emitting a warning for the non-build-related validation failure. This will catch schema errors such as allowed_outbound_hsots = ...:

$ spin build
Building component self-req-test with `cargo build --target wasm32-wasi --release`
    Finished `release` profile [optimized] target(s) in 0.03s
Finished building all Spin components
Warning: The manifest has errors not related to the Wasm component build. Error details:
TOML parse error at line 18, column 1
   |
18 | allowed_outbound_hsots = []
   | ^^^^^^^^^^^^^^^^^^^^^^
unknown field `allowed_outbound_hsots`, expected one of `source`, `description`, `variables`, `environment`, `files`, `exclude_files`, `allowed_http_hosts`, `allowed_outbound_hosts`, `key_value_stores`, `sqlite_databases`, `ai_models`, `build`, `tool`

HOWEVER.

The problem in #2520 relates to variable name validation, and that does not happen at manifest load, or even lockfile creation, but in the trigger when it tries to resolve the variables. Checking that in the loader would require additional validation logic in the loader, similar to how we validate kebab-ids. Nothing stops us adding that validation logic, but it's not there now, and for this PR I didn't want to extend load-time validation (with who knows what consequences) but only run the existing load-time validation at build time. If we decide to add variable name validation at load time then spin build with this PR will inherit it.

[mikkelhegn]

The approach in this PR looks good to me. (Not a code review :-))

[lann]

Perhaps rename this component_build_configs_with_error or something? And update the docstring?

[itowlson]

I'm not mad keen on that name (and can't think of a fully expressive one that isn't awkward) but have updated the doc string.