step.request_parse: support application/x-www-form-urlencoded body parsing

[intel352]

Current Behavior

step.request_parse with parse_body: true only supports JSON bodies. When the Content-Type is application/x-www-form-urlencoded, the step reads the body bytes via io.ReadAll() then calls json.Unmarshal(), which fails silently. As a result:

• output["body"] is never populated — the form field data is lost
• The body stream is consumed and unavailable for subsequent steps (e.g. step.webhook_verify)

This breaks webhook integrations that send form-encoded POST bodies, such as Twilio.

Requested Behavior

When parse_body: true is set and the Content-Type header is application/x-www-form-urlencoded, step.request_parse should:

1. Parse the body using url.ParseQuery() instead of json.Unmarshal()
2. Expose each form field in output["body"] as a string (first value) or []string (multiple values)
3. Cache the raw body bytes in pc.Metadata["_raw_body"] so subsequent steps (e.g. step.webhook_verify) can still access the original payload

Example

Given a Twilio webhook POST with body:

Body=Hello&From=%2B15551234567&To=%2B15559876543&MessageSid=SM1234

After step.request_parse with parse_body: true:

steps.<name>.body.Body       = "Hello"
steps.<name>.body.From       = "+15551234567"
steps.<name>.body.To         = "+15559876543"
steps.<name>.body.MessageSid = "SM1234"

And pc.Metadata["_raw_body"] should contain the original raw bytes so step.webhook_verify (which needs to re-read the body for HMAC-SHA1 validation) can still function.

Workaround

A custom step.form_parse step that reads _raw_body from pipeline metadata and parses via url.ParseQuery().