Add a new Redis input plugin

[fujimotos]

This adds support for fetching messages from Redis Pub/Sub. This
basically allows Fluentd to act as a data stream subscriber.

Here is an example configuration that uses this plugin:

<source>
  @type redis
  channel test
  tag redis
  <parse>
    @type none
  </parse>
</source>

Now you can start publishing events to the 'test' channel:

redis.cli> PUBLISH test aiueo
redis.cli> PUBLISH test abcabc

... which will produce events like this:

2021-09-30 09:13:58.887849790 +0000 redis.test: {"message":"aiueo"}
2021-09-30 09:13:58.979078488 +0000 redis.test: {"message":"abcabc"}

Signed-off-by: Fujimoto Seiji [email protected]