Stricter parsing of aggregations.

 - Only one aggregation type is allowed per aggregation definition.
 - Return an error in the parsers when an unknown field is encountered.

Close #4464
[DOCS] Added the official PHP client to the community page.
[DOCS] Documented index.codec.bloom.load for #4525
[DOCS] Corrected bloom loading docs
[DOCS] Fixed community links to official clients