Implement `from_unixtime` function

[andygrove]

Is your feature request related to a problem or challenge? Please describe what you are trying to do.
In the SO post https://stackoverflow.com/questions/72888852/extract-year-month-day-from-unix-timestamp-column-in-rust-datafusion-dataframe/72941102#72941102 the user needed help with translating a unixtime to a timestamp. The solution now is to use a cast and this is verbose and not obvious. It would be nice to have a specific from_unixtime function.

Describe the solution you'd like
See https://spark.apache.org/docs/2.3.0/api/sql/index.html#from_unixtime for Spark's version

Describe alternatives you've considered
A clear and concise description of any alternative solutions or features you've considered.

Additional context
Add any other context or screenshots about the feature request here.

[waitingkuo]

@andygrove
would you mind giving me some hint? I'm still new to this project. thanks~

Is this the right place to begin with https://github.com/apache/arrow-datafusion/blob/master/datafusion/expr/src/built_in_function.rs#L150 ?

[comphead]

@waitingkuo https://github.com/apache/arrow-datafusion/blob/master/CONTRIBUTING.md#how-to-add-a-new-scalar-function this can be some start for you

[comphead]

#2324 please check this PR also

[waitingkuo]

thank you @comphead !

[waitingkuo]

@comphead i finally figured out how to add a new function, thank you~

@andygrove
the current implementation only support Int64 as input value, should we support floating number and utf-8 as well?
the pyspark version has the second argument to render user preferred time format, should we support that in this pr?

[MrDataPsycho]

@waitingkuo Can you please refer me where you have added that code? I only found 2 references of the function:

1. expr_fn.rs
2. timestamp.rs

But could not find the actual function. I want to add some example usage doc in the function. As it looks empty without any example: crates doc . Or if you can tell how I should add some doc. The contribution guide says the function should be added into datetime expression. But could not find that function there.

[waitingkuo]

@DataPsycho
it's located here
https://github.com/apache/arrow-datafusion/blob/45fc415daa7028559ef3477e53a184a114149f9e/datafusion/physical-expr/src/functions.rs#L155-L171

you could follow this to find all related functions
https://github.com/apache/arrow-datafusion/blob/master/docs/source/contributor-guide/index.md#how-to-add-a-new-aggregate-function