An enum MiniDescriptor simply encodes a set of valid `int32_t` values, so that the protobuf parser can test whether a given enum value is known or not.
The format implemented here is novel and needs to be documented. In short, the format is:
1. base92 values 0-31: 5-bit mask indicating presence or absence of the next five enum values.
2. base92 values 60-91: varint indicating skip over a region of enum values.
Negative enum values are encoded as their `uint32_t` equivalent.
PiperOrigin-RevId: 442892799