Audigent RTD configurable per-bidder segment mappings (#5903)

* configurable per bidder segment mappings, optional segment caching, documentation update

* rubicon->appnexus in docs

* apply fpd convention, remove publisher defined mappers, add unit tests, fixes

* segment mapping arr->dict

* add fpd data to bids

* update audigent tests

* openrtb fpd data descriptions, update docs, more tests

* update docs, update integration example

* integration update

* documentation table update

* add examples of adding and overriding segment mappers

* fix documentation

* add contact verbiage

* requestParams object type

* brand rtd provider

* update appnexus segment function

* support numerical segments for appnexus

* no default appnexus segments

* update url

* update gpt example

* conditionally set bid segments for generic

* fixes for integration example

* update integration example

* add appnexus handler override