* Make CEF key name mapping case-insensitive

There's some case inconsistency in CEF docs (i.e. C6a4Label). Better to
ignore case when mapping keys to full names.

* Add missing custom CEF extensions

This adds:
 - `deviceCustomIPv6Address2(Label)`: Only 1, 3 and 4 were expected.
 - `flexNumber[12](Label)`: These two alternative custom numbers were
   dropped after V23 of the spec, but still used by some vendors.

[Maybe unnecessary] changes:

 - Changed the case of `DeviceCustomNumber2` from uppercase as
   documented) to lowercase to align with the other fields.

* CEF module: Support Check Point devices

This adds a new ingest pipeline and fields to populate from Check Point
CEF logs.

Closes elastic#16041

(cherry picked from commit f6fde2e)