feat: match labels FromEnvironmentFieldPath

[tenstad]

Description of your changes

It is currently difficult to lookup EnvironmentConfigs based on values from environment. For example fetching one environment config (or resource, using function-extra-resources) and then fetch another environment config based on values in the first one.

The PR extends the types of matchLabels to include FromEnvironmentFieldPath.

It reuses the valueFromFieldPath and fromFieldPathPolicy fields used by FromCompositeFieldPath, as they did not contain composite, but could instead create valueFromEnvironmentFieldPath and fromEnvironmentFieldPathPolicy.

Tests are updated to also test FromEnvironmentFieldPath wherever FromCompositeFieldPath is tested.

I have:

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

[phisco]

missing error handling here

[tenstad]

Yeah, I don't check the ok from v, ok := f[key] as there is no guarantee that the environment key exists in the context. And we want the resulting map to just be empty if it does not exist exit. Both .GetStructValue() and .AsMap() are nil-safe, so there is no need to do conditionally call them. Added a comment to clarify.

[tenstad]

What do you think is best? Skipping the check, or performing it as in #119 (comment)

[tenstad]

Could also just do the if check I guess, avoiding the comment and potential confusion.

Suggested change

	// Skip key presence check as the context key is not required and map conversion is nil safe.
	envCtx, _ := request.GetContextKey(req, FunctionContextKeyEnvironment)
	env := envCtx.GetStructValue().AsMap()
	var env map[string]any
	if envCtx, ok := request.GetContextKey(req, FunctionContextKeyEnvironment); ok {
	env = envCtx.GetStructValue().AsMap()
	}