Allow forcing HTTP/2 on template-level

[Nirusu]

Currently, -force-http2 is implemented as a global flag across all templates.
However, certain templates might want to specifically request HTTP/2 to check for HTTP/2 specific headers/trailers or protocols (e.g. gRPC).

Currently, even when a template uses raw HTTP and specifies HTTP/2 like this:

http:
  - raw:
    - |
        GET /whatever HTTP/2
        Host: {{ Hostname }}
        Content-Type: application/grpc

Nuclei defaults to HTTP/1.1, which is a bit annoying for this use-case.
Meanwhile, -force-http2 is advertised as breaking a lot of stuff, so you cannot mix templates that require HTTP/2 with ones that require HTTP/1.1.

I saw that this was already requested once: #5164
But the Discord link doesn't work for me, so I don't know what the outcome of the discussion here was.