Adds `testing_commitBlockV1`. It is the write companion of `testing_buildBlockV1`:
it builds a block from the provided payload attributes and transactions on
top of the current canonical head, inserts it, and sets it as the new
head, returning the new head hash.
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Co-authored-by: MariusVanDerWijden <m.vanderwijden@live.de>
This is a copy of #34721 but against `master` (rather than
`bal-devnet-3`), as requested by @jwasinger, since the slotnum logic now
exists on `master` as well.