Support default properties for bindings

[mbogoevici]

Suggested solution: add @Value("${spring.cloud.stream.rabbit.binder.default.autoBindDlq}") on binding properties and add additional-spring-configuration-metadata.json for completion.

[ilayaperumalg]

This means we allow both the defaultproperties to be specified along with the actual binding properties themselves (for instance having both spring.cloud.stream.rabbit.binder.default.autoBindDlq and spring.cloud.stream.rabbit.bindings.<channelName>.producer.autoBindDlq)?

Given we already have the default values defined for these producer/consumer properties at the field level, I would like to understand the rational behind supporting the default properties along with the binding properties (which could override the default ones). @mbogoevici

[mbogoevici]

Given we already have the default values defined for these producer/consumer properties at the field level, I would like to understand the rational behind supporting the default properties along with the binding properties (which could override the default ones).

That pretty much describes the motivation. If we had multiple inputs or outputs, it'd be useful to specify different defaults than the ones that the field is initialized with globally (while retaining the ability to do per-binding configuration).

[ilayaperumalg]

Since the binder specific binding properties (for example RabbitProducerProperties and RabbitConsumerProperties) aren't ConfigurationProperties themselves and they are instantiated explicitly, having @Value won't resolve automatically since it requires BeanPostProcessor to kick in or we need to explicitly resolve the properties using PropertyPlaceholder support.

This featured is deferred for now.