This change removes the XPackExtension mechanism in favor of
SecurityExtension that can be loaded via SPI and doesn't need
another (duplicate) plugin infrastructure
Original commit: elastic/x-pack-elasticsearch@f39e62a040
* Security Realms: Predictable ordering for realms
To have predictable ordering of realms, by having secondary
sorting on realm name resulting in stable and consistent documentation.
Documentation update describing how ordering of realms is determined.
Testing done by adding unit test for the change, ran gradle clean check locally.
relates elastic/x-pack-elasticsearch#3403
Original commit: elastic/x-pack-elasticsearch@98c42a8c51
Simon Willnauer
Yogesh Gaikwad
debadair