Add Support for JSON Log format

[wilwade]

We've had a request from some to add an option for JSON logs.

Not sure if this is simple, but there might be another alternative which is to produce documentation on how to parse with logstash or such.

Mentioned a few times in Substrate Issues, but no movement there:

• Structured logging paritytech/substrate#7019
• Configurable logger paritytech/substrate#7357
• Logging middleware paritytech/jsonrpc#614