Navigation Block: Allow for transparent submenu background

[cuemarie]

What problem does this address?

Some users may want their Navigation Block's submenu to have a transparent background, to match the transparent background of the navigation block itself in it's default state. For example:

However, it's currently not possible to do so without the use of additional CSS. When there is not a background color set, the submenu background defaults to white (at least on them: TT3), and it's not possible to set it as transparent from the color picker in the editor.

What is your proposed solution?

Adding a transparency slider to the custom color picker would solve this problem.

Possibly related? #42170 (I tried the tactics in that discussion here, though, and wasn't able to make that work.)

Originally requested here: Automattic/wp-calypso#80367