Chore: add immutable validity for mutable vectors

[connortsui20]

Tracking Issue: #5028

[a10y]

is it better to do this or to provide a is_valid(index: usize) accessor? why would you want to access the validity before freezing?

[connortsui20]

I need it to access child validities for ListVector validation (need to check that there are no nulls in offsets and sizes), and tbh this seems like a nice thing to have in general

[codecov]

Codecov Report

❌ Patch coverage is 21.05263% with 30 lines in your changes missing coverage. Please review.
✅ Project coverage is 85.04%. Comparing base (d2272a3) to head (c3f870f).
⚠️ Report is 1 commits behind head on develop.

Files with missing lines	Patch %	Lines
vortex-vector/src/binaryview/vector_mut.rs	0.00%	3 Missing ⚠️
vortex-vector/src/bool/vector_mut.rs	0.00%	3 Missing ⚠️
vortex-vector/src/decimal/generic_mut.rs	0.00%	3 Missing ⚠️
vortex-vector/src/decimal/vector_mut.rs	0.00%	3 Missing ⚠️
vortex-vector/src/fixed_size_list/vector_mut.rs	0.00%	3 Missing ⚠️
vortex-vector/src/null/vector_mut.rs	72.72%	3 Missing ⚠️
vortex-vector/src/primitive/generic_mut.rs	0.00%	3 Missing ⚠️
vortex-vector/src/primitive/vector_mut.rs	0.00%	3 Missing ⚠️
vortex-vector/src/struct_/vector_mut.rs	0.00%	3 Missing ⚠️
vortex-vector/src/vector_mut.rs	0.00%	3 Missing ⚠️

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