Add cache collector

[lublak]

For complexer work it would be nice to have something like this:

cache_collector:
  resource: cache_resource
  key: ""
  init: root = []
  append_check: this.process == "work"
  append_map: root = this.old.append(this.new)
  flush_check: this.process == "end"
  flush_deletes: true
  flush_map: root = this

logic for each message (process):

• get cache by resource config
• check if append_check returns true on current message
• if append_check bloblang returns true
  • read the key from key config in cache as current value
  • if not exists call the bloblang init function which is now the current value
  • call append_map bloblang function with "this.old" (the current value) and "this.new" (the current message), the return value is then the current value
  • store the value into the cache
  • return nothing from processing (no message)
• if flush_check bloblang true
  • read the key from key config in cache as current value
  • if not exists call the bloblang init function which is now the current value
  • if flush_deletes is true the cache key gets removed
  • call flush_map bloblang function which returns from processing the new message (replaces the current message)
• if both checks are not true
  • just return the message as it is