ipc_helpers: Add PushEnum() member function to ResponseBuilder

Allows pushing strongly-typed enum members without the need to always
cast them at the call sites.

Note that we *only* allow strongly-typed enums in this case. The reason
for this is that strongly typed enums have a guaranteed defined size, so
the size of the data being pushed is always deterministic. With regular
enums this can be a little more error-prone, so we disallow them.

This function simply uses the underlying type of the enum to determine
the size of the data. For example, if an enum is defined as:

enum class SomeEnum : u16 {
  SomeEntry
};

if PushEnum(SomeEnum::SomeEntry); is called, then it will push a
u16-size amount of data.