[Feature]: Add custom upgrade testing framework

[SatabdiG]

#202

Add Custom Upgrade Test Framework

Implements a flexible upgrade test framework for the CloudFoundry provider, enabling specialized validation during provider upgrades beyond standard resource health checks.

What's New

Custom Upgrade Test Framework (upgrade_test_framework.go)

• builder API for creating upgrade tests with custom pre/post-upgrade validations
• Configurable timeouts, resource directories, and test phases
• Per-test ProviderConfig management to avoid CRD ownership conflicts

Example Custom Test (space_external_name_upgrade_test.go)

• Validates Space external-name follows UUID format
• Ensures external-name annotations don't change during upgrades

New Test Infrastructure

• CreateProviderConfig() helper for per-test ProviderConfig creation
• LoadUpgradePackages() for version resolution

New Make Targets

• make test-upgrade-base - Run baseline tests only
• make test-upgrade-custom - Run custom tests only
• make test-upgrade-compile - Verify compilation
• make test-upgrade-help - Show usage examples

Testing

✅ Baseline upgrade test (v0.3.2 → v0.3.3): PASS
✅ Custom external-name test: PASS

Documentation

Updated upgrade-testing.md with quickstart guide
Usage examples for creating custom tests
Environment variable reference

Note

Follows patterns from crossplane-provider-btp for consistency across SAP providers.