agent: requests to replica should have dfx version and ic-ref client info

[gobengo]

via: https://dfinity.slack.com/archives/CGA566TPV/p1600410292001800

Why?

• It will help replica team track usage of different dfx versions across different nets

[gobengo]

@nomeata @hansl fyi

[gobengo]

Would a custom header for this need to be defined in ic-ref spec? Or is there already a place to 'extend' or sandbox new features?

[gobengo]

@nomeata Are there any issues in other repos or tools that this can link to? That we we could coordinate making changes at similar times.

[nomeata]

We could document this in ic-ref. But since it's meant to be informative, but not actually relevant for the behavior of the system, I'd say just go ahead and include such a header, without a full-fledged design process

[gobengo]

HTTP header?

[hansl]

Dfx-Version: 1.2.3

WDYT?

[hansl]

Also, Accept-Ic-Spec: 1.2.3, 4.5.6, 7.8.9