[Feature]: add support for templates in in HTTP headers & body (and perhaps in general)

### Feature Request

It should be pretty obvious what I was trying to do here:


I figured putting `{{var_name}}` in e.g. Headers or Body inputs would cause an input to show up, so I could connect it to context.

Of course, there's pretty natural workaround, just use a text node:


It's just slightly less convenient - it would be nice if you didn't need to manually template outside of the HTTP call block to achieve this.

In fact, wouldn't it be nice if this feature just worked consistently everywhere, with any text input? 🙂

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