[Enhancement] Access Flat_object Subfields Using Docvalues

[mingshl]

Is your feature request related to a problem? Please describe.
in 2.7.0 release, Flat_object Field does not support searching subfields in PainlessScript.

Describe the solution you'd like
enable searching in PainlessScript, we will need to direct the fielddatabuilder to fetch docvalues within the two stringfields in memory

[PromenadePierce]

Is there any estimation on when this is expected to be released? This functionality is core to our use-case and migration to AWS OpenSearch

[mingshl]

Hi @lukas-vlcek, you mentioned in #7137 that you would like to take a look at this issue, do you have estimation for the release in planning?

[kkewwei]

@lukas-vlcek , If you haven't started doing it yet, I'd also love to try it.