[Refactor] Modularise code

[EdricChan03]

The code could do with some modularisation...

Current plans

• Move Ktor StringValues extension functions to separate module
• Remove Ktor requirement from HTTP client
  • Abstract away Ktor Headers/Parameters/StringValues (ended up copying over the code for now :)
  • Remove Ktor requirement from system-headers/parameters
• Move system-headers/parameters to separate module