feat: add arithmetic function "power" with decimal type

[anshuldata]

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[anshuldata]

@EpsilonPrime I don't have write access to branch. Can you kindly merge ? Thanks

[EpsilonPrime]

As this PR is somewhere between a new feature and a change to an existing one I'd like to get another approval for this PR before we merge it. I'll check with the other folks at the Substrait community meeting on Wednesday and will likely merge it then.

[jacques-n]

@anshuldata can you rebase?

[anshuldata]

@anshuldata can you rebase?

rebased it