ENH: Add Plugin API for 'default providers' and BeanScopeBuilder.provideDefault() methods

[rbygrave]

This allows plugins to add a 'default provider' (as a @Secondary) that can be used when nothing else provides that dependency.

For example, Jsonb can have a plugin to use () -> Jsonb.builder().build() as a provider like:

/**
 * Plugin for avaje inject that provides a default Jsonb instance.
 */
public final class DefaultJsonbProvider implements io.avaje.inject.spi.Plugin {

  @Override
  public Class<?>[] provides() {
    return new Class<?>[]{Jsonb.class};
  }

  @Override
  public void apply(BeanScopeBuilder builder) {
    builder.provideDefault(Jsonb.class, () -> Jsonb.builder().build());
  }
}

[SentryMan]

So if a dependency provides a plugin, I don't need to define @InjectModules explicitly? Or did I misread this entirely?

[rbygrave]

Yes, you got it.

Jsonb and the Validator are 2 good examples. For something to be a plugin:

1. It only provides one component
2. The component it provides must not have any dependency
3. The component it provides is not named/qualified and has @Secondary priority so will be used only when nothing else provides the same component

So we'd use a Plugin rather than a full blown avaje-inject module with @InjectModules etc because with the plugin it ends up as approximately 2 lines of code and a full blown avaje-inject module instead looks a bit more like serious overkill.

[SentryMan]

Yeah I figured. I'm getting a dependency not provided error on my javalin repo when I use the new validator. I guess the service loader can't load the plugins at compile time. #263 fixes this.