text: Title: [new]: `xml_extract(xml, xpath)`
  Body: ### Check the idea has not already been suggested
  
  - [X] I could not find my idea in [existing issues](https://github.com/unytics/bigfunctions/issues?q=is%3Aissue+is%3Aopen+label%3Anew-bigfunction)
  
  ### Edit the title above with self-explanatory function name and argument names
  
  - [X] The function name and the argument names I entered in the title above seems self explanatory to me.
  
  ### BigFunction Description as it would appear in the documentation
  
  Extract part of xml using xpath.
  
  Inspired from https://github.com/stankiewicz/bigquery-xml-parser-udf
  
  ### Examples of (arguments, expected output) as they would appear in the documentation
  
  `xml_extract("<customer><name>Paul</name></customer>", "/customer/name")`
  --> `Paul`