feat: header in http processor

[harshdoesdev]

Users can pass request headers as processor arguments. Header keys are keys that match this pattern: $header-[key-name]$. For example, $header-authorization$ can be used to set the Authorization header of a request (as suggested by @Arpita-Jaiswal).

-- import: fastn/processors as pr


-- create-post-response response:
$processor$: pr.http
method: POST
url: /blog/new/
$header-authorization$: Bearer auth-token


-- ftd.text: $response.message

[amitu]

@harshdoesdev lets make this: $header-authorization$: Bearer auth-token instead. If tomorrow someone wants to send header-name or something as normal POST parameter they will end up sending wrong header.