Support dead letter queue on sinks

In the context of an event processor a dead letter queue can mean a number of things. We can already support content based DLQs using transforms to route certain events to a secondary sink when they aren't suitable for our primary sink.

However, it would also be nice to support dead letter queuing by chaining sinks as fallback options, where failed sends can be routed under certain circumstances (rather than retrying in an infinite loop, or being dropped completely).

It'd be cool to be able to do something like this:

[sinks.foo]
  inputs = [ "somewhere" ]
  type = "something"
  [sinks.foo.dlq]
    after_retries = 3

[sinks.bar]
  inputs = [ "foo.dlq" ]
  type = "something_else"

And since we're just producing output from sinks here there's no reason we can't add transforms in there as well:

[sinks.foo]
  inputs = [ "somewhere" ]
  type = "something"
  [sinks.foo.dlq]
    after_retries = 3

[transforms.baz]
  inputs = [ "foo.dlq" ]
  type = "clean_up"

[sinks.bar]
  inputs = [ "baz" ]
  type = "something_else"

Start by locating the sink retry and failure-handling paths, then review how sink and transform inputs are represented in configuration. Define the fallback behavior for exhausted retries, including how events reach a chained sink or transform, and add coverage for the TOML examples and failure cases.