Passing version to server-side adapters

[bretg]

The use cases for passing PBS version through the ecosystem are troubleshooting and feature sensitivity. e.g. "if version > x then expect A". Note that analytics adapters already have access to the PBS version.

The proposal is to pass ext.prebid.server.version to adapters. Alternately, we could make it available as a global function or variable.

It was noted that this is a tiny bit more CPU and network for a field that's rarely used. We could do it only in test/debug mode.

Open for public comment.

[bretg]

This is tracked by #1818