chore: add utilities to LanceBuffer

[westonpace]

This is the first in a series of PRs that eventually reworks the v2 encoders to use DataBlock. This PR doesn't add any public facing capability on its own but merely adds a few useful utilities to LanceBuffer.

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 77.84810% with 35 lines in your changes missing coverage. Please review.

Project coverage is 79.29%. Comparing base (1e6ee60) to head (46e2a8a).
Report is 1 commits behind head on main.

Files	Patch %	Lines
rust/lance-encoding/src/buffer.rs	77.84%	34 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2784      +/-   ##
==========================================
- Coverage   79.30%   79.29%   -0.02%     
==========================================
  Files         227      227              
  Lines       68268    68426     +158     
  Branches    68268    68426     +158     
==========================================
+ Hits        54141    54257     +116     
- Misses      11006    11037      +31     
- Partials     3121     3132      +11     

Flag	Coverage Δ
unittests	79.29% <77.84%> (-0.02%)	⬇️

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[broccoliSpicy]

huh, Vec::from array copies the stack located array data into heap?

[westonpace]

Ah, you are right. I read that "it moves the values into the Vec" and thought that meant zero-copy but it just means the items inside the Vec are moved and not cloned.

[westonpace]

I changed the impl into a copy_array method and updated the comment. I'd like to keep all From implementations zero-copy (again, trying to make it obvious when a copy of data happens)

[broccoliSpicy]

LGTM!