refactor(partition-mapper): rename ToXXXMapper to StartOfXXXMapper

[Lee-W]

Why

During b2 testing, I received feedback that ToXXXMapper was still confusing.

What

rename ToXXXMapper to StartOfXXXMapper

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[Lee-W]

@uranusjr Since we're now renaming it to StartOfXXXMapper now, we probably can include the full timestamp? or should we? so for a StartOfMonthMapper, it currently generates a partition key like 2027-01

[uranusjr]

I think it’s fine to truncate the key, people probably aren’t interested in those extra zeros.

[jason810496]

Technically approve, I just double-checked for all the renaming, but I didn't go deep with the discussion thread.