This adds "enterprise" as an acceptable type for a license loaded
through the PUT _license API.
Internally an enterprise license is treated as having a "platinum"
operating mode.
The handling of License types was refactored to have a new explicit
"LicenseType" enum in addition to the existing "OperatingMode" enum.
By default (in 7.x) the GET license API will return "platinum" when an
enterprise license is active in order to be compatible with existing
consumers of that API.
A new "accept_enterprise" flag has been introduced to allow clients to
opt-in to receive the correct "enterprise" type.
Backport of: #49223
47e5e34f42