Agentic AI: Support CONNECTOR_HTTP_PROXY* settings for A2A HTTP connections

[maff]

Is your feature request related to a problem? Please describe.

For the HTTP calls done by the A2A connectors, we should honor the CONNECTOR_HTTP_PROXY_* settings for compliance and security reasons.

Describe the solution you'd like

When connector proxy settings are configured, configure a proxy on the respective A2A HTTP client.

Describe alternatives you've considered

Additional context

See https://docs.camunda.io/docs/next/components/connectors/protocol/rest/#configure-a-proxy-server-in-self-managed