[SPARK-3379] Implement 'POWER' for sql

[xinyunh]

Add support for the mathematical function "POWER" within spark sql. Test-cases included.
Spitted from [SPARK-3176].

[AmplabJenkins]

Can one of the admins verify this patch?

[SparkQA]

Can one of the admins verify this patch?

[marmbrus]

ok to test

[SparkQA]

QA tests have started for PR 2252 at commit ebe1da9.

• This patch does not merge cleanly!

[nchammas]

For reference, how does this implementation of POWER() differ from SQL Server's?

Syntax
POWER ( float_expression , y )
Arguments
float_expression: Is an expression of type float or of a type that can be implicitly converted to float.
y: Is the power to which to raise float_expression. y can be an expression of the exact numeric or approximate numeric data type category, except for the bit data type.
Return Types
Returns the same type as submitted in float_expression. For example, if a decimal(2,0) is submitted as float_expression, the result returned is decimal(2,0).

[marmbrus]

Good comparison @nchammas, It odes not look like this implementation currently preserves the input type or supports decimal.

[SparkQA]

QA tests have finished for PR 2252 at commit ebe1da9.

• This patch fails unit tests.
• This patch does not merge cleanly!

[chenghao-intel]

Nit: Let's check if base_v is null first and then compute exponent_v, probably performance gains in some cases.

[marmbrus]

Thanks for working on this, but we are trying to clean up the PR queue (in order to make it easier for us to review). Thus, I think we should close this issue for now and reopen when its ready for review.