ENH: Add prefix, prefix_sep, dummy_na, and dtype args to StringMethods get_dummies()

[aaronchucarroll]

Adding these args makes StringMethods get_dummies() similarly powerful to pd.get_dummies(), but usable in the common scenario that the strings are separated by some separator.

• [x] closes #47872
• [x] Tests added and passed if fixing a bug or adding a new feature
• [x] All code checks passed.
• [x] Added type annotations to new arguments/methods/functions.