Implement writing to Context

[ytsarev]

Description of your changes

• Implement writing to Context
• Support previous Context merging
• Match with common function layout
• Add example for render

Testing

• Unit

go test -v -cover .
...
    --- PASS: TestRunFunctionSimple/MergeContext (0.00s)
...
    --- PASS: TestRunFunctionSimple/SetContext (0.00s)
...

• Render test

go run . --insecure --debug
...
cd examples/default/context
make
crossplane render --verbose xr.yaml composition.yaml functions.yaml -rc
2025-01-16T16:24:06+01:00	DEBUG	Starting development runtime. Remember to run the function manually.	{"function": "kcl-function", "target": "localhost:9443"}
---
apiVersion: example.crossplane.io/v1beta1
kind: XR
metadata:
  name: example
status:
  conditions:
  - lastTransitionTime: "2024-01-01T00:00:00Z"
    reason: Available
    status: "True"
    type: Ready
---
apiVersion: render.crossplane.io/v1beta1
fields:
  contextField: contextValue <-- Expected context data was properly written
  moreComplexField:
    test: field
kind: Context

Fixes #134

I have:

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

[Peefy]

Amazing Job! LGTM!