feat: improve e2e testing tooling and documentation

[rawkode]

Major improvements to the end-to-end testing infrastructure:

Testing Infrastructure:

• Add automatic payment method ID fetching from RedisCloud API
• Create templated test examples with dynamic payment method injection
• Add cluster/test/setup.sh script for comprehensive test environment setup
• Add datasource.yaml for dynamic value injection in tests
• Support environment variable configuration via .envrc

Documentation Updates:

• Restructure TESTING.md with clear quick-start instructions
• Add comprehensive payment method configuration section
• Document environment variable setup with .envrc.example
• Update CI-TESTING.md to reflect current CI/CD workflows
• Clean up references to removed scripts in all docs

Cleanup:

• Remove obsolete scripts (quick-start.sh, setup-podman.sh, test-provider.sh)
• Consolidate functionality into Makefile targets
• Add proper .gitignore entries for test artifacts
• Remove untracked files (provider-aws/, kubeconfig, etc.)

Developer Experience:

• Add devenv.nix configuration for development environment
• Add rediscloud-payment-methods and rediscloud-first-payment-method-id make targets
• Improve error handling and user feedback in setup scripts
• Simplify credential management with environment variables

This refactoring makes the e2e testing process more robust and user-friendly, particularly for testing Pro subscription features that require payment methods.

Description of your changes

Fixes #

I have:

• Read and followed Crossplane's contribution process.
• Run make reviewable test to ensure this PR is ready for review.

How has this code been tested

[Copilot]

Pull Request Overview

This PR significantly improves the end-to-end testing infrastructure and developer experience for the RedisCloud Crossplane provider. It removes obsolete scripts in favor of standardized Makefile targets, adds automatic payment method ID fetching for Pro subscription testing, and provides comprehensive documentation updates.

Key changes:

• Remove legacy testing scripts and consolidate functionality into Makefile targets
• Add automatic payment method ID fetching from RedisCloud API for Pro subscription tests
• Implement templated test examples with dynamic value injection via setup script
• Restructure documentation with clear quick-start instructions and environment setup

Reviewed Changes

Copilot reviewed 19 out of 20 changed files in this pull request and generated 2 comments.

Show a summary per file

File	Description
scripts/test-provider.sh	Removes 362-line legacy testing script
scripts/setup-podman.sh	Removes obsolete podman setup script
scripts/quick-start.sh	Removes quick-start wrapper script
cluster/test/setup.sh	Adds comprehensive test environment setup with payment method ID fetching
cluster/test/examples/*.yaml.tmpl	Adds templated test examples for dynamic value injection
Makefile	Adds RedisCloud API integration targets and improves e2e testing
TESTING.md	Major restructure with clear workflows and payment method configuration
.envrc.example	Adds example environment configuration file
devenv.nix	Adds crossplane-cli and gettext packages

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