[Agent] Rule extracting items from array

[michalpristas]

ExtractListItemRule extract items with specified name from a list of maps.
The result is store in a new array.

Example:
Source:

streams:
  - name: MySQL error log
    input:
      type:	file
      path:	/var/log/mysql/error.log
  - name: MySQL access log
    input:
      type:	file
      path:	/var/log/mysql/access.log
  - name: MySQL metrics
    input:
      type: mysql
      host: localhost
      port: 3306

Called

ExtractList("streams", "input", "inputs")

For extracting input fields from streams list and storing it in new list named inputs

result:

streams:
  - name: MySQL error log
    input:
      type:	file
      path:	/var/log/mysql/error.log
  - name: MySQL access log
    input:
      type:	file
      path: /var/log/mysql/access.log
  - name: MySQL metrics
    input:
      type: mysql
      host: localhost
      port: 3306
inputs:
  - type: file
    path: /var/log/mysql/error.log
  - type: file
    path: /var/log/mysql/access.log
  - type: mysql
    host: localhost
    port: 3306

cc #15690

[elasticmachine]

Pinging @elastic/ingest (Project:fleet)

[ph]

Just a small suggestion.

[ph]

looking at the method, its a problem with all the cases statement of the switch case, we could do:

Suggested change

	}
	return unpack(fields, r)

[michalpristas]

we cant return just but i will change it to less chatty way