List Channel Capabilities

What each registered channel can do (edit_message, delete_message, reactions, mentions, ...), keyed by channel name.

The inbox reads it once and shows only the actions a channel supports: an Edit button on an Avito message could only end in a 400 (BACKEND-2HG). The map depends only on the build, so it is built once; a map with a hole (one adapter failed to construct) is answered but rebuilt no sooner than a minute later, so neither a flood nor a broken adapter turns every request into constructing every adapter.