Add prometheus response format to endpoints

[n8maninger]

Adds an internal Prometheus encoder for encoding responses in the Prometheus sample format.

Also adds a response path param to some endpoints to support different output formats (i.e. http://localhost:9980/api/settings?response=prometheus)

Supported formats:

• prometheus
• json

Supported endpoints:

• GET /state/host
• GET /state/consensus
• GET /settings
• GET /metrics
• GET /wallet

[n8maninger]

@bustedware I didn't add all of the endpoints, but this is the direction we should be going wrt supporting Prometheus within hostd. If there are any changes you need to make, you can either let me know or make them yourself after this is merged.