Support loading context when running crossplane beta render

[negz]

Description of your changes

Context can be loaded from either values, or files. Either way, the value must be JSON. This could be used to inject "the environment" for testing function-patch-and-transform once crossplane-contrib/function-patch-and-transform#17 is merged. It could also be useful for developing new Functions that read from the environment.

Examples:

$ crossplane beta render xr.yaml comp.yaml fns.yaml \
  --context-values=apiextensions.crossplane.io/environment='{"key": "value"}';other-key='"stringvalue"' \
  --context-files=apiextensions.crossplane.io/environment=env.json

Note that context values are not merged. If a key appears in both --context-files and --context-values, the latter wins. If a key appears multiple times in the same flag, the last appearance wins.

I have:

• Read and followed Crossplane's contribution process.
• Added or updated unit and E2E tests for my change.
• Run make reviewable to ensure this PR is ready for review.
• Added backport release-x.y labels to auto-backport this PR, if necessary.
• Opened a PR updating the docs, if necessary.