fix: feature gate utils fn using optional rust_decimal dep

[lmmx]

Hi there, hope this small PR is not overlooking anything: I tried to build the trunk and it failed due to one dependency miss, the rust_decimal import is actually an optional dependency.

The utils function it is used in is only called by code gated by the arrow feature flag, so it is equivalent to feature-gate the util too. I moved the use rust_decimal::Decimal; line down into a direct reference to it in the parameter, to avoid repeating the feature gate directive, and likewise for anyhow::Result (to avoid the unused import warning).

• It entered the code base 3 months ago here: c7d6ee3

[lmmx]

• This is now also contributed in Bump rusqlite 0.33 -> 0.37; r2d2_sqlite 0.26 -> 0.31 #845 (which also bumps DB dependency versions), feel free to accept that one instead if it's ready or merge separately

[wangxiaoying]

Thanks @lmmx for contributing! I will close this one for now and redirect to #845 for this change.