feat: Support spark base64 function

[leoluan2009]

Which issue does this PR close?

Closes #419 .

Rationale for this change

What changes are included in this PR?

How are these changes tested?

[leoluan2009]

@viirya Help to start CI, thanks

[leoluan2009]

@viirya help to start CI and review code, thanks

[viirya]

Looks like there are incompatible results compared with Spark.

[leoluan2009]

Looks like there are incompatible results compared with Spark.

datafusion use Basic type of base64 scheme while spark use MIME type. After search the source of https://github.com/marshallpierce/rust-base64 which is used by datafusion for base64 function, I found this lib do not support MIME type of base64 scheme, so it may difficult to add Spark-compatible base64/unbase64 function. @viirya do you have some good ideas？

[viirya]

Could we find another library supporting that? We can implement custom base64 function in Comet.

[leoluan2009]

Could we find another library supporting that? We can implement custom base64 function in Comet.

I will do some research and have a try, thanks