[Merged by Bors] - feat(RingTheory/Polynomial/Hilbert): Polynomial.exists_unique_hilbertPoly and Polynomial.hilbertPoly_mul_one_sub_pow_add

[Merged by Bors] - feat(RingTheory/Polynomial/Hilbert): Polynomial.exists_unique_hilbertPoly and Polynomial.hilbertPoly_mul_one_sub_pow_add #48958