fix: generate code: curl: use --data-urlencode

[cvmocanu]

When the body is "Form URL encoded", the generated curl code should:

• either manually encode the value of the field
• or use --data-urlencode which does the encoding itself (not supported on old versions of curl)

Currently, if we have a field called field with a value a&=b, the value is sent as-is.

With data, the following is sent (obviously incorrect): field=a&=b
With --data-urlencode, the following is sent (correct): field=a%26%3Db

[cvmocanu]

probably this is the root cause: Kong/httpsnippet#260

[helloanoop]

@sreelakshmi-bruno please have this slotted in our dev sprint for Jan 2025. We want to make some improvements in the Generate Code UI and lets spend some time refining it in Dec.