[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?

[mingshl]

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

of course. done

[ryn9]

@mingshl @lukas-vlcek

Can we please get (and maybe prioritize :) ) the addition of the "ignore_malformed" option for flat_object?

Reference:
https://opensearch.org/docs/latest/field-types/

When set to true , and a document is received that is not valid json, the document will not be rejected, rather the field would ignored.
IE - the field would not be indexed, and added the the "_ignored" array

This would help in situation like that referenced in the following issue, where we cannot 100% control if a valid json object is being provided.
#8250
While this would not make the field searchable , when the option is set, at least the document would not be rejected.

[kkewwei]

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

[lukas-vlcek]

@kkewwei Feel free to jump on it.
I currently do not have any spare capacity to work on it.

[msfroh]

@kkewwei -- Thanks a lot for picking this one up!

Once your fix is in (or maybe there's some overlap?) you might want to take a look at #11635.

For that one, I think we can clean up some (most?) of the query logic from FlatObjectFieldType -- a lot of it (I think) would become "If this query is qualified with a path, then delegate to valueAndPathFieldType, else delegate to valueFieldType".

[kkewwei]

@msfroh get it, it's a good idea, It can simply the code, I will try to implement it with you suggestion.