feat: add proxy_ssl_context to Configuration for a separate proxy TLS context
Fixes #2387.
Configuration gains proxy_ssl_context: Optional[ssl.SSLContext] = None, passed through to urllib3.ProxyManager in RESTClientObject. This lets the HTTPS proxy's TLS cert be validated independently of the destination (K8s API server) settings - the "HTTPS Proxy + HTTPS Destination" scenario urllib3 already supports natively.
kubernetes/client is generated from kubernetes-client/gen; companion PR kubernetes-client/gen#308 adds the same patch there. This PR applies it to the already-generated files so it's usable now.
Verification: kubernetes/e2e_test/test_issue_2387_proxy_ssl_context.py spins up a mock HTTPS proxy and destination server with independent self-signed CAs (no live cluster needed). Without proxy_ssl_context: fails with CERTIFICATE_VERIFY_FAILED. With it set to trust the proxy's CA: succeeds, destination still validated separately.
Added `proxy_ssl_context` to `Configuration`, letting users provide a separate SSL context for HTTPS proxy TLS verification, independent of the destination TLS settings.