Add ThreadSafe client caching to IdentityAwareBuilder

[siddimore]

Description of your changes

Adds thread-safe client caching to IdentityAwareBuilder to avoid creating redundant Kubernetes clients for the same ProviderConfig identity. This reduces connection overhead when multiple Objects reference the same ProviderConfig.

Changes:

• Added cachedClient struct wrapping client.Client with createdAt timestamp
• Added cache (sync.Map) and cacheTTL fields to IdentityAwareBuilder
• KubeForProviderConfig() now checks cache before creating clients
• Added cacheKeyForProviderConfig() helper generating stable keys from identity source
• Default TTL: 30 minutes (change to configurable)

Fixes #430

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

Defined new tests:

• Builder construction
• Cache key generation
• Cache key uniqueness
• Client expiry behavior
• Cache hit with valid client