Define a proper Pydantic model for the values returned by `HLAAlgorithm.combine_standards_stepper`

[rhliang]

Currently this method returns a generator which yields values of type tuple[tuple[int, ...], int, tuple[str, str]]. These values are:

• the "mushed-together" sequence, in binary form (i.e. where each nucleotide is represented as an integer between 0 and 15);
• the mismatch count of this combined standard sequence to the sequence we're analyzing; and
• the names of the alleles going into this combined standard.

This works fine but now that we're working with this method more directly (in the BBLab tool) it's not very friendly to deal with. Define a proper Pydantic model for these return values.