Implement `power` function

[alamb]

Is your feature request related to a problem or challenge? Please describe what you are trying to do.
We would like to compute a power function (like 2**4 = 2 * 2 * 2 * 2) in datafusion

@matthewmturner asks for it here: #147 (comment)

Describe the solution you'd like

Implement the power function as described in https://www.postgresql.org/docs/14/functions-math.html

power ( a numeric, b numeric ) → numeric
power ( a double precision, b double precision ) → double precision

a raised to the power of b

power(9, 3) → 729

Describe alternatives you've considered
It may also be interesting to investigate adding support for a**b as another way to write the same thing

Additional context
This is probably a good ticket to learn about the datafusion codebase and its function implementations -- one could follow the example of one of the existing functions in

https://github.com/apache/arrow-datafusion/blob/master/datafusion/src/physical_plan/functions.rs

[matthewmturner]

ill work on this

[matthewmturner]

ive started looking into this and i think it makes sense to add power / power_scalar compute kernels to arrow first then expose it in physical_plan/expressions/binary.rs and physical_plan/expressions/functions.rs

@alamb ok with that?

[alamb]

@matthewmturner I think power is more like a function rather than an operator -- so in that case I think you could just add power to BuiltinScalarFunction (and then fill in the implementation):

https://github.com/apache/arrow-datafusion/blob/5668be78a9ccbf5469e9e95ad070920ca5d105ba/datafusion/src/physical_plan/functions.rs

[matthewmturner]

@alamb ok happy to do that.

but just to show you where i was coming from, i saw both C++ (https://arrow.apache.org/docs/cpp/compute.html#arithmetic-functions) and arrow2 (https://github.com/jorgecarleitao/arrow2/blob/main/src/compute/arithmetics/basic/pow.rs) had kernels for power. so i thought we would want to do something similar and not have datafusion implement something that could / should be done downstream.

[alamb]

@matthewmturner -- I don't have a strong preference.