Add --report-json report type

[jcamiel]

This is a specific issue to add a new type of report --report-json to tackle #1907 #1969.

The proposal:

$ hurl --test --report-json my-directory *.hurl

Will create a report under my-directory

$ tree my-directory
my-directory
├── index.json
└── store
    ├── 9c01db72-b2fa-442c-a454-ed8bd00a8145-response.json
    ├── 9f02cc8d-deb4-4830-8bf3-def1d93311a3-response.xml
    └── c9ea8402-ffab-41da-aaef-80d4766ec332-response

index.json is a JSON array containing the same JSON as if we'd run --json on each file, plus an additional field response that reference the response file in store. This is quite similar to how is structured --report-html.