feat(float): implement footer

Problem: Now way to show text at the bottom part of floating window
  border (a.k.a. "footer").

Solution: Allows `footer` and `footer_pos` config fields similar to
  `title` and `title_pos`.

runtime/lua/vim/_meta/api_keysets.lua

@@ -108,6 +108,8 @@ error('Cannot require a meta file')
 --- @field border? any
 --- @field title? any
 --- @field title_pos? string
+--- @field footer? any
+--- @field footer_pos? string
 --- @field style? string
 --- @field noautocmd? boolean