feat: setup a docker environment for the connector

[Joel-hanson]

This pull request introduces improved TLS configuration options for the Kafka Connect Elasticsearch Sink Connector, and adds a comprehensive Docker-based test environment to validate these changes. The documentation (README.md) is updated to reflect the new configuration options and provide detailed instructions for building, running, and testing the connector.

TLS and Security Configuration Enhancements

• Added support for specifying keystore and truststore types (es.tls.keystore.type, es.tls.truststore.type) and the option to skip SSL verification (es.tls.skip.verification) in both documentation and example configuration (README.md, config/elastic-sink.properties). [1] [2]
• Updated the configuration table in the documentation to clarify and expand TLS-related options, including new fields for keystore/truststore type and SSL verification skipping.

Test Environment and Validation

• Added a new docker-test-environment directory containing a Dockerfile, docker-compose.yml, test configurations, and scripts to automate building, deploying, and validating the connector with the new TLS options. [1] [2] [3] [4]
• Updated the README.md with a new section detailing how to use the Docker-based test environment, including prerequisites, test steps, and manual testing instructions.

Documentation and Usability Improvements

• Enhanced build and run instructions in the README.md with explicit shell command blocks and clarified steps for Maven build, repository cloning, and connector execution. [1] [2]
• Improved formatting and clarity in configuration tables and code blocks throughout the documentation for better readability and user experience. [1] [2] [3]

General Updates

• Minor formatting and link improvements, such as updating the license URL to angle brackets and cleaning up markdown formatting. [1] [2] [3] [4] [5]

These changes make it easier to securely configure the connector for TLS, validate new options in a controlled environment, and follow improved documentation for setup and testing.