Compatible with OpenSearch and OpenSearch Dashboards Version 1.2.0

• Add new protocol for visualization response format (#251)

• Add conversion support for datetime as per https://github.com/kylepbit (#267)
• Optimized type converting in DSL filters (#272)

• Create 1.2 release notes (#268)
• Update notice files (#269)
• Update license header (#282)
• Updated PPL user manual with relevance function limitations (#283)

• Bumps version to 1.2 (#254)