perf: Use Cow in get_format_string in FFI_ArrowSchema

[andygrove]

Which issue does this PR close?

N/A

Rationale for this change

We're using a lot of FFI in Comet and are looking for any performance wins we can get.

ffi_arrow_schema_try_from
                        time:   [497.34 ns 499.01 ns 500.56 ns]
                        change: [-11.996% -11.725% -11.490%] (p = 0.00 < 0.05)
                        Performance has improved.

What changes are included in this PR?

Use Cow in get_format_string to reduce some string allocations.

Are there any user-facing changes?

No