subspec: implement Poseidon2 spec with tests #4

Job	Run time
Lint and format checks - Python 3.12	16s
Tests - Python 3.12 on ubuntu-latest	10s
Tests - Python 3.13 on macos-latest	14s
Tests - Python 3.12 on macos-latest	13s
Lint and format checks - Python 3.13	17s
Tests - Python 3.13 on ubuntu-latest	9s
	1m 19s

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