set default_codec doc attribute

bump to 4.2.0

[skip ci] update gemspec summary

enable use of metadata fields in `add_field` decorations

By performing event decorations after the metadata field has been populated
we enable the supported `add_field` decoration to reference metadata fields.

Resolves logstash-plugins#76

Fixes logstash-plugins#78

Fix broken link

Fixes logstash-plugins#77

Version bump

For elastic/logstash#7993

[ci skip]

bump patch level for doc generation

Fix scrolling requests by using json formatted bodies

This makes scrolling incompatible with ES 1.x
NOTE: It's already documented that version 5.x of this
gem isn't compatible with ES 1.x

Upgrade es-ruby client to use 5.0.2 which supports correct content-type 

Fixes logstash-plugins#55

Fixes logstash-plugins#57

Use manticore client

Fixes logstash-plugins#54