messages: support topic/subscriber filter policies

[derekperkins]

It should be possible for message tables to conditionally subscribe to topics, based on message metadata. For prior art, see subscription filter policies for SNS.
https://docs.aws.amazon.com/sns/latest/dg/sns-subscription-filter-policies.html

[derekperkins]

As I've been thinking about it, it would really be awesome if we could support filtering with SQL statements rather than building our own filter conditionals. Since we're likely going to be executing this in vtgate, I'm not sure that'd be possible, but that'd really lean into our strengths.