feat: Async search client

[adilansari]

• User friendly search request builder. Introduced following capabilities:

SearchRequest.newBuilder("my search").build();
SearchRequest.newBuilder("my search").withSearchFields("first_name", "last_name").build();
SearchRequest.newBuilder("my search").withFacetFields("first_name", "last_name").build();

• Updated readme docs
• Async search client

Checks

• Unit tests
• Javadocs publishing passes without error
• Coverage for new features