UI/nvim_ui_attach(): add override option

Before now, Nvim always degrades UI capabilities to the lowest-common
denominator. For example, if any connected UI has `ext_messages=false`
then `ext_messages=true` requested by any other connected UI is ignored.

Now `nvim_ui_attach()` supports `override=true`, which flips the
behavior: if any UI requests an `ext_*` UI capability then the
capability is enabled (and the legacy behavior is disabled).

Legacy UIs will be broken while a `override=true` UI is connected, but
it's useful for debugging: you can type into the TUI and observe the UI
events from another connected (UI) client. And the legacy UI will
"recover" after the `override=true` UI disconnects.

Example using pynvim:

    >>> n.ui_attach(2048, 2048, rgb=True, override=True, ext_multigrid=True, ext_messages=True, ext_popupmenu=True)
    >>> while True: n.next_message();

test/functional/api/vim_spec.lua

@@ -1284,7 +1284,7 @@ describe('API', function()
     end)
     it('returns attached UIs', function()
       local screen = Screen.new(20, 4)
-      screen:attach()
+      screen:attach({override=true})
       local expected = {
         {
           chan = 1,
@@ -1299,6 +1299,7 @@ describe('API', function()
           ext_messages = false,
           height = 4,
           rgb = true,
+          override = true,
           width = 20,
         }
       }
@@ -1308,6 +1309,7 @@ describe('API', function()
       screen = Screen.new(44, 99)
       screen:attach({ rgb = false })
       expected[1].rgb = false
+      expected[1].override = false
       expected[1].width = 44
       expected[1].height = 99
       eq(expected, nvim("list_uis"))