[Enhancement] Add Open Parameters to Flat_object Field Type

[mingshl]

Is your feature request related to a problem? Please describe.
Flat objects currently do not support open parameters. Looking forward to add open parameter to increase better search results.

Describe the solution you'd like
The following open parameters setting are useful to consider to add in, including normalizer, docValues, ignoreAbove, nullValue, similarity, and depthlimit.

normalizer: allow lowercase, uppercase
docValues: allow to set docValues to be false
ignoreAbove: allow to set if the length of a field is go above certain limit then ignore the document.
nullValue: allow to set nullValue to others
similarity: allow to set similarity setting
depthlimit: allow to set maximum depth limitation to the JSON document

Additional context
Add any other context or screenshots about the feature request here.

[mingshl]

Hi @lukas-vlcek, are you planning to take up this issue?

[lukas-vlcek]

Sure, I can take this one. Thank you!

And I am happy to take a look at the other tickets later too if they will be still available (#7136, #7138).

[mingshl]

I will take up #7136, but #7138 is also help wanted.

[mingshl]

@lukas-vlcek If you decide to work on this issue, would you like to assign yourself in this issue?

[lukas-vlcek]

@mingshl I can not assign myself (have no privs to do that). Can you please assign me?