Crossplane v1-compatible build of function-extra-resources

[jakubramut]

Description of your changes

What

• Protocol: revert to the legacy extra_resources gRPC fields (read/call/write) instead of the v2-only resources/required_resources. This is the only functional change - everything else (max-recv flag, custom context key, sorting, selectors) is untouched.
• Package metadata: constraint >=v2.0.0-0 → >=v1.15.0-0; removed the capabilities: [required-resources] block.
• Namespace guard: if Crossplane doesn't advertise capabilities (v1.x or <v2.2), a selector with namespace set now fails fast - on v1 the namespace filter is ignored server-side and would silently match across all namespaces.
• Plus two guard unit tests and a README compatibility note.

Why

Upstream main (v0.3.0+) moved to the v2-only protocol fields, so it silently no-ops on Crossplane v1. This stream restores v1 support while staying dual-compatible: it uses the deprecated fields, which Crossplane v2 still resolves.

How has this code been tested

• go build, go test, golangci-lint (CI-pinned v2.6.2): all pass.
• E2E on kind, Crossplane 1.20.0: function installs; label-selected EnvironmentConfigs land in a custom context key and reach a downstream go-templating step (two-pass legacy round-trip); namespace selector yields the guard's Fatal event.
• E2E on kind, Crossplane 2.3.4: same build installs and the same composition works (dual-compat); a namespace-scoped selector is honored server-side (guard stays silent).
• --max-recv-message-size: verified it raises the function's request-recv limit (function accepted a 5.4 MB request that the 4 MB default rejected).

I have:

• Read and followed Crossplane's contribution process.
• Added or updated unit tests for my change.

[phisco]

Constraint and README look right now. One thing left.

[phisco]

Longer term, return requirements.resources when HasCapability(req, CAPABILITY_REQUIRED_RESOURCES) and the legacy field otherwise, merging both on read — then only the v1 leg breaks when the deprecated fields go. Beyond this PR's scope.

[jakubramut]

@phisco,
thank you for the review - agreed with your comment - that's the right long-term shape. Keeping it out of this PR as you suggested - happy to follow up separately.