Implement API to fetch details about blocks

[diox]

API side of #13681

The API only needs to fetch details about a specific block, not list them. It needs to support fetching by add-on guid + version or legacy block ID.

[AlexandraMoga]

I've verified the blocklist api on -dev and I have the following results:

• a block can be queried by the add-on guid or by our internal block id (exposed in the block detail URL)
• the api response exposes the internal block id, created and modified dates, min-max version range, add-on guid, block reason and URL
• when trying to fetch a deleted block, the api responds with a 404

@eviljeff has the initial request to support fetching by the legacy block ID been dropped?

[eviljeff]

@eviljeff has the initial request to support fetching by the legacy block ID been dropped?

yes, for now #13681