Add HTTP and gRPC engine ping support

[stsingh]

Summary

• add ping() and ping(int) to ChalkClient
• implement HTTP /ping and gRPC QueryService.Ping transports
• route gRPC requests correctly when the API server is used as the query server
• accept the CLI CHALK_ENVIRONMENT variable while preserving CHALK_ACTIVE_ENVIRONMENT
• tolerate newer fields in the shared Chalk token file

Verification

• TestPing.testHttpPing passed
• TestPing.testGrpcPing passed
• TestConfigLoader.TestLoadAllTokens passed
• live Booking HTTP ping through https://api.chalk.ai succeeded and echoed 42
• full suite: 73 passed, 1 skipped; 4 existing integration classes failed initialization because live integration-test credentials were not supplied