Support enum signals for enum classes that are generated at runtime

[DiamondJoseph]

As part of instantiating (e.g. a PandaA) from its PVI tree, we have a series of enumeration signals for which we do not know the possible values they can take. We should be able to instantiate those signals, and access a generated Enum class which allows iterating through all of the possible choices that that underlying PV supports. This type is not available for static type checking.

[coretl]

Should use the Literal syntax defined in #249 (comment)

[DiamondJoseph]

Isn't the Literal syntax for #238 and this is GeneratedChoice?

[coretl]

Isn't the Literal syntax for #238 and this is GeneratedChoice?

Yes, you're right. However I think we should drop GeneratedChoices as it's not pickleable, and instead treat these as str datatype, but put choices in the output of get_descriptor. This is still introspectable enough for us, while still being a simple datatype to pass around and serialize, and looks a lot like the output of #238