Add initial Decimal32/Decimal64 implementation

[zeroshade]

Related: apache/arrow#43956

Rationale for this change

Widening the Decimal128/256 type to allow for bitwidths of 32 and 64 allows for more interoperability with other libraries and utilities which already support these types. This provides even more opportunities for zero-copy interactions between things such as libcudf and various databases.

What changes are included in this PR?

This PR contains the basic Go implementations for Decimal32/Decimal64 types, arrays, builders and scalars. It also includes the minimum necessary to get everything compiling and tests passing without also extending the acero kernels and parquet handling (both of which will be handled in follow-up PRs).

Are these changes tested?

Yes, tests were extended where applicable to add decimal32/decimal64 cases.