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.lua

@@ -1551,11 +1551,18 @@ function vim.api.nvim_open_term(buffer, opts) end
 ---                   specified by character: [ ["+", "MyCorner"], ["x",
 ---                   "MyBorder"] ].
 ---
----               • title: Title (optional) in window border, String or list.
----                 List is [text, highlight] tuples. if is string the default
----                 highlight group is `FloatTitle`.
----               • title_pos: Title position must set with title option.
----                 value can be of `left` `center` `right` default is left.
+---               • title: Title (optional) in window border, string or list.
+---                 List should consist of `[text, highlight]` tuples. If
+---                 string, the default highlight group is `FloatTitle`.
+---               • title_pos: Title position. Must be set with `title`
+---                 option. Value can be one of "left", "center", or "right".
+---                 Default is `"left"`.
+---               • footer: Footer (optional) in window border, string or
+---                 list. List should consist of `[text, highlight]` tuples.
+---                 If string, the default highlight group is `FloatTitle`.
+---               • footer_pos: Footer position. Must be set with `footer`
+---                 option. Value can be one of "left", "center", or "right".
+---                 Default is `"left"`.
 ---               • noautocmd: If true then no buffer-related autocommand
 ---                 events such as `BufEnter`, `BufLeave` or `BufWinEnter` may
 ---                 fire from calling this function.

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