Add PxL UDFs for standard math and string operations

[aimichelle]

PxL is used to process and transform data collected by Pixie. In its current state, we have already have many functions implemented for most basic data processing (see: https://docs.px.dev/reference/pxl/)
However, we are still missing functions that users may want to execute on their data. In particular, this includes math and string operations such as itoa/atoi, string replace, and more.

[philkuz]

Note: we already have atoi (https://docs.px.dev/reference/pxl/udf/atoi/)

For potential contributors: We want to ensure that UDFs are performant and don't error out during execution. Sometimes that means "obvious" UDFs will need to be carefully implemented. The last thing we want is someone spending valuable time writing a UDF and having it rejected.

Here are some UDFs sorted on likelihood of being accepted:

Likely to be accepted

• px.itoa: INT64 -> STRING
• px.make_upid: INT64, INT64, INT64 -> UINT128 (UPID)

Difficult
Please reach out to the #pxl channel in Slack if you want to contribute these.

• px.float_to_string(df.float_col, '%.4f')
• px.string_to_float(df.string_col, default_float_value)
• px.string_replace(df.string_col, 'to_replace', 'replace_with')

[Chuxiaof]

Hello! I would like to contribute to this project for GSOC'22. How can I contact you for further details? Thanks!