The prefix for the fields and headers can be a list instead of a string

It is now possible to give the configurable string prefix for the fields and headers.

• "add.fields.prefix" -> String with the default value of "__"
• "add.headers.prefix" -> String with the default value of  "__"

This makes all the fields and headers to have the same prefix.

We can change this to a list of strings.
Then each individual field and header can have different prefixes. 
This might also be useful when anyone want to only prefix for few fields/headers and not all. 

•  "add.fields.prefix" -> list of strings.
  • When given the # of prefixes MUST be equal to # of fields given in "add.fields". We can configure empty prefix as well.
•  "add.headers.prefix" -> list of strings.
  • When given the # of prefixes MUST be equal to # of headers given in "add.headers". We can configure empty prefix as well.