Add validation for reranker processor parameters

[martin-gaievski]

For the reranker processor, there are no checks of number of context fields user can define for the processor when:

• defining processor as part of the search pipeline
• using query_text_path as part of search request

This brings risks of system instability in case very long string is passed in any of those two places.

I suggest we use some limit on number of fields for document_fields, let's start from 50 but that number is discussible. For the query_text_path we can limit the max level of nested field to not exceed one that set by index.mapping.depth.limit setting.

[martin-gaievski]

@HenryL27 please check this request

[HenryL27]

ack

[HenryL27]

any other instabilities?

[martin-gaievski]

any other instabilities?

not as of now, but let's keep this issue open, I may add more stuff in coming days. this is based on a feedback I'm getting from security reviews

[HenryL27]

k. for now making a setting for RERANKER_MAX_DOC_FIELDS ("plugins.neural_search.reranker_max_document_fields")