Switch to Upbound DevEx

[ytsarev]

Description of your changes

• Migrated from traditional Crossplane configuration with patch-and-transform functions to modern Upbound DevEx approach
• Converted composition to pipeline-mode with embedded KCL functions for Flux GitOps deployment
• Added comprehensive testing infrastructure with composition and e2e tests

I have:

• Read and followed Upbound's contribution process.
• Run make reviewable to ensure this PR is ready for review.
• Added backport release-x.y labels to auto-backport this PR, as appropriate.

How has this code been tested

Composition tests

up test run tests/*
  ✓   Parsing tests
  ✓   Checking dependencies
  ✓   Generating language schemas
  ✓   Building functions
  ✓   Building configuration package
  ✓   Pushing embedded functions to local daemon
  ✓   Assert flux-test
SUCCESS:
SUCCESS: Tests Summary:
SUCCESS: ------------------
SUCCESS: Total Tests Executed: 1
SUCCESS: Passed tests:         1
SUCCESS: Failed tests:         0

e2e tests

 up test run tests/* --e2e
        === COMMAND
        /bin/sh -c ${KUBECTL} wait --for=delete xflux.gitops.platform.upbound.io/test-flux-instance --timeout 1h15m0s
    | 17:57:39 | delete | Assert Deletion  | SCRIPT    | DONE  |
    | 17:57:39 | delete | Assert Deletion  | TRY       | END   |
    | 17:57:39 | delete | @chainsaw        | CLEANUP   | SKIP  |
=== NAME  chainsaw
    | 17:57:39 | chainsaw | @chainsaw | CLEANUP   | SKIP  |
--- PASS: chainsaw (0.00s)
    --- PASS: chainsaw/delete (3.47s)
PASS
2025/06/20 17:57:39 Tests Summary:
2025/06/20 17:57:39 - Passed: 1
2025/06/20 17:57:39 - Failed: 0
2025/06/20 17:57:39 - Skipped: 0
SUCCESS:
SUCCESS: Tests Summary:
SUCCESS: ------------------
SUCCESS: Total Tests Executed: 1
SUCCESS: Passed tests:         1
SUCCESS: Failed tests:         0

[kaessert]

LGTM!