Make the promoted-builds-plugin DSL extension for job-dsl-plugin extensible: Automatically Generated DSL

The current DSL extension is static and needs to be modified/improved whenever a new feature is added to promoted-builds-plugin.
By the extensible nature of this plugin (it provides the PromotionCondition ExtensionPoint for third parties to implement), this makes the DSL extension requirement to be dynamic depending on available plugins ; this doesn't scale well with the current static DSL extension.
job-dsl-plugin 1.46 introduced the Automatically Generated DSL feature to address this issue on job-dsl-plugin itself by the use of javaposse.jobdsl.dsl.ExtensibleContext / javaposse.jobdsl.dsl.ContextType at the DSL level (model introspection done by helpers such as javaposse.jobdsl.plugin.structs.DescribableContext).

The promoted-builds-plugin DSL extension should make use of those DSL authoring tool to automatically discover available PromotionCondition.