[B+C] Added iron golem holding flower methods. Adds BUKKIT-5440

##### The Issue:
Iron Golem randomly holds flower to give it to random villager (pathfind magic). It only last 20 seconds (400 game ticks), but there is no way to get or set holding of flower.

##### Justification:
There is no way to get if iron golem is holding flower or set iron golem to hold flower.

##### PR Breakdown:
This PR adds a method to get if iron golem is holding flower and method to set iron holem to hold flower.

##### CraftBukkit PR:
[Bukkit/CraftBukkit#1348](https://github.com/Bukkit/CraftBukkit/pull/1348)

##### JIRA Ticket: 
[JIRA Ticket #5440](https://bukkit.atlassian.net/browse/BUKKIT-5440)
Implementation details shouldn't really be exposed in JavaDocs. Possibly something like `The flower will remain for a limited time` is better?