[JENKINS-19843] Provides an extension point for other plugins to support rebuild plugin.

This is an improved approach for #14.

When a new plugin with its own parameter value comes, we need to add a new jelly file for that plugin to rebuild plugin (as in bf501fb).
This means rebuild plugin must know all Jenkins plugins, which is not realistic.

This pull request add a feature for other plugins to support rebuild plugin.
If a plugin with its own parameter wants to work with rebuild plugin, it does as followings:
1. Define a new class implementing `RebuildParameterProvider`.
2. Define `getRebuildPage()`, which returns a hint to access a jelly file for that parameter value.
3. Annotate it with `@Extension`.

By specifying `option=true` to `@Extension`, plugins can be loaded even without rebuild plugin.
If there is no `RebuildParameterProvider` suitable for a parameter value class, rebuild plugin works as now.