Split up impl for float -> decimal, add spark rapids code

[pmattione-nvidia]

Split up the float -> decimal conversion implementation in order to support both pyarrow and spark conversions. The spark code can live in spark-rapids-jni instead, if we can expose the internal functions it calls to it.

Checklist

• I am familiar with the Contributing Guidelines.
• New or existing tests cover these changes.
• The documentation is up to date with these changes.