[ew] section metadata to filter which blocks the block library allows adding

To support migrating crosswalk customers to EW, we need to support filtering which blocks can be added to which sections.

Implementaiton proposal:
* Add section metadata called filter with the value being a name of a filter
* In the block library, add a new filters tab where the key is the name of the filter and value a csv list of blocks

No files or tests are named. Start by locating the section metadata handling and the block library entry point, then trace how available blocks are assembled. Done means sections can declare a named filter and the block library exposes a filters tab whose values are CSV block lists and uses them to control which blocks may be added.

Written by the indexing model from the issue text.