feat: support decimal for math functions: power (#18032)

## Which issue does this PR close?

- Closes #18031

## Rationale for this change

<!--
Why are you proposing this change? If this is already explained clearly
in the issue then this section is not needed.
Explaining clearly why changes are proposed helps reviewers understand
your changes and offer better suggestions for fixes.
-->

## What changes are included in this PR?

<!--
There is no need to duplicate the description in the issue here but it
is sometimes worth providing a summary of the individual changes in this
PR.
-->

## Are these changes tested?

- Additional unit tests
- Existing SLT tests

## Are there any user-facing changes?

No

Author: theirix