Support 'enterprise' license type

[ph]

Elasticsearch teams want to add a new license type called "enterprise", we need to add this new type to the list of supported types.

beats/x-pack/libbeat/licenser/types.go

Lines 11 to 18 in b089094

	const (
	OSS LicenseType = iota // Open source
	Trial // Trial
	Basic // Basic
	Standard // Standard
	Gold // Gold
	Platinum // Platinum
	)

The Enterprise type will be a superset of the Platinum License.

NOTE: This is a breaking change for older best and the following comment need to be resolved elastic/elasticsearch#48510 (comment)

Elasticsearch issues elastic/elasticsearch#48510

[bytebilly]

Thanks for creating this @ph!

Do you already know if this change could happen in 7.6? This would help in a better understanding of the migration process, even if it's not strictly required.

As far as I can guess, the required change in Beats will not create any breaking change (even if we'll have the breaking change in Elasticsearch), so there are no concerns to add it. Is this correct?