Accept gzip encoded content on webhooks

Webhooks currently accept JSON data, which will be sent for processing to the workflow.

But the webhooks do NOT accept gzip-encoded data.

Gzipping is a typical way to compress payloads on the wire.

Note that we're NOT talking about attaching a gzip file and extracting it somewhere. We're just talking about how the request body content is encoded.

But it doesn't work today! We get:

Plug.Parsers.ParseError) malformed request, a Jason.DecodeError exception was raised with message "unexpected byte at position 0: 0x1F"
        (plug 1.18.1) lib/plug/parsers/json.ex:95: Plug.Parsers.JSON.decode/3
        (plug 1.18.1) lib/plug/parsers.ex:341: Plug.Parsers.reduce/8

Reproduction Steps

Use CURL to send a JSON object into your favourite webhook:

curl -v -X POST \
  http://localhost:4000/i/ab1801c2-3af0-40cf-8f0c-b743a0afce9b \
  --json @tmp/payload.json \
  --compressed

The --compressed flag will gzip the body

Start with the webhook request path and the Plug.Parsers.JSON error shown in lib/plug/parsers/json.ex and lib/plug/parsers.ex, then reproduce it with the provided curl command using --compressed. Done means the gzip-encoded JSON webhook request is accepted and processed without the reported parse error; check for a relevant request test in the repository.