[Filebeat] [Meta] Netflow input improvements

[adriansr]

A few issues are being identified in the released NetFlow input (beta):

• Fix field snake-case conversion bug (postNATSourceIPv4Address => post_nast_ource_ipv4_address vs post_nat_source_ipv4_address) [Filebeat] [Netflow] fix field name conversion to snake case #10950
• Support custom field definitions. Allow users to add and/or override NetFlow V9/IPFIX fields using files in custom field definitions in Logstash format. [Filebeat] NetFlow input support for custom field definitions #10945
• Add support for Cisco ASA devices.: NetFlow fields missing / acl_id_asa datatype. [Filebeat] NetFlow: Add Cisco ASA fields #11201
• Add missing fields for vendors (Streamcore, Palo Alto, Netstream, ...) (see testdata)

[kaiyan-sheng]

@adriansr Is this issue still valid?

[elasticmachine]

Pinging @elastic/siem (Team:SIEM)

[adriansr]

Closing as we added most of the missing fields from vendors.