Field generator - add support for IP grok pattern

[jasonslater2000]

https://github.com/elastic/beats/blob/master/filebeat/generator/fields/fields.go#L41

the grok pattern "IP" is not included in the map - can we add it so that it maps to ES "ip" type?

var (
types = map[string]string{
"group": "group",
"DATA": "text",
"GREEDYDATA": "text",
"GREEDYMULTILINE": "text",
"HOSTNAME": "keyword",
"IPHOST": "keyword",
"IPORHOST": "keyword",
"LOGLEVEL": "keyword",
"MULTILINEQUERY": "text",
"NUMBER": "long",
"POSINT": "long",
"SYSLOGHOST": "keyword",
"SYSLOGTIMESTAMP": "text",
"LOCALDATETIME": "text",
"TIMESTAMP": "text",
"USERNAME": "keyword",
"WORD": "keyword",
}
)

[elasticmachine]

Pinging @elastic/integrations-services (Team:Services)