MINOR: vars: implement dump_all_vars() sample fetch

MINOR: vars: implement dump_all_vars() sample fetch

This patch implements dump_all_vars([scope],[prefix]) sample fetch
function that dumps all variables in a given scope, optionally
filtered by name prefix.

Output format: var1=value1, var2=value2, ...
- String values are quoted and escaped (", , \r, \n, \b, \0)
- All sample types are supported via sample_convert()
- Scope can be: sess, txn, req, res, proc
- Prefix filtering is optional

Example usage:
  http-request return string %[dump_all_vars(txn)]
  http-request return string %[dump_all_vars(txn,user)]

This addresses GitHub issue #1623.