Use string_view in CookieParser::before_handle parsing

[Sightem]

This PR refactors CookieParser::before_handle to use std::string_view for parsing the Cookie request header, avoiding extra std::string allocations during slicing and trimming

Tested against the original code with around 20 inputs, confirming identical output maps and rejection behavior.
Closes #1023

[Sightem]

I tested the implementation against the old one using these:

std::vector<std::string> test_cookies = {
   // Basic cases
   "key=value",
   "key1=value1; key2=value2",
   " key=value ",
   "key = value ; nextkey = anothervalue ",
   // Quoted values
   "key=\"value\"",
   "key=\"\"",
   " key = \" spaced value \" ",
   "key=\"value with ; inside\"",
   "key=\"value with = inside\"",
   "key=",
   "key=;next=val",
   "key= ; next=val",
   // Edge cases
   "",
   "key",
   "=", 
   "=value",
   " key = ",
   "key=value;"
   ";key=value",
   "key1=value1;;key2=value2",
   "key= value ",
   "key=\" value \"",
   "utf8key=你好世界; other=val",
   "special=!@#$%^&*()_+",
   "SID=abc; something=\"\"; key3=val3"
};