feat: Add support for tls type and skip verification

[Joel-hanson]

This pull request introduces several improvements and new features to the Kafka Connect Elasticsearch sink connector, primarily focused on enhancing TLS configuration flexibility and updating documentation. The most significant changes are the addition of support for custom keystore/truststore types and the option to skip SSL verification, along with corresponding updates to documentation and configuration files.

• Add support for truststore and keystore tls type change
• Add support for tls skip verification

TLS Configuration Enhancements

• Added support for specifying keystore and truststore types (JKS, PKCS12, etc.) via new configuration options es.tls.keystore.type and es.tls.truststore.type, with defaults set to JKS. These are now handled in both the connector and writer classes and passed to the Jetty SSL context. [1] [2] [3] [4] [5]
• Introduced a new configuration option es.tls.skip.verification to allow skipping SSL verification for Elasticsearch connections. This is configurable and logged as a warning if enabled. [1] [2] [3] [4] [5]

Documentation and Example Updates

• Updated the README.md to document the new TLS configuration options, including keystore/truststore types and SSL verification skipping. Improved formatting and added code blocks for clarity. [1] [2] [3] [4]
• Updated example configuration in config/elastic-sink.properties to include the new TLS options and clarify their usage.

Dependency Update

• Bumped Apache Kafka dependency version from 3.6.1 to 4.0.0 in pom.xml for improved compatibility and access to new features.

Minor Documentation Improvements

• Made several formatting and clarity improvements in README.md, such as code block formatting, improved list readability, and fixed license URL formatting. [1] [2] [3] [4] [5] [6]

These changes collectively improve the connector's security configuration flexibility and make it easier for users to understand and configure TLS options.