Add ID() to the Block interface

[samstarling]

At incident.io, we often iterate over the contents of Slack views, and often want to add the block ID to various bits of observability (logs, traces). Doing this is quite difficult, because the Block interface doesn't allow easy access to the ID — we have to cast each possible kind of block to pull this out.

This PR introduces an ID() function to the Block interface, making this kind of thing much easier.

I added some tests along the way, and also noticed that CallBlock doesn't allow the ID to be set. This doesn't affect us, but you might wish to fix that.

[calebmckay]

Fixes #1358.

This definitely seems like a good feature to have when parsing a BlockSet from a JSON payload. I'm not sure how often we'd want to set a BlockID manually from the New*Block functions, but I guess the main problem is that it's not currently consistent across block types.

[samstarling]

@calebmckay Nice, thanks! I hadn't seen that issue. Can I ask what the path to getting this reviewed/approved is? Just want to make sure I've not missed anything. But yes, you're right — it's useful to (for example) get a payload from Slack and be able to inspect the block IDs without having to do a bunch of type conversion.

[calebmckay]

It looks like @lorenzoaiello is the primary active maintainer right now, so they'll need to review and merge.

[samstarling]

Hey @parsley42, I was wondering, are you the current maintainer here?

[parsley42]

No, you want @lorenzoaiello

[nlopes]

Although technically this can be a breaking change, I think people should be relying on what we provide so am going to consider this as part of a non-breaking change batch.