[server-timing] first run at spec draft

https://w3c.github.io/server-timing/

- metrics are communicated via Server-Timing header field
- each metric is represented as a ServerEntry (extends PerformanceEntry)
- metrics are accessed via same API as NT/RT: same interface extensions,
global buffer, etc.

For a hands-on overview checkout:
- https://w3c.github.io/server-timing/#examples
- https://w3c.github.io/server-timing/#use-cases

Would appreciate any thoughts and/or feedback! Does it look reasonable, any
objections to proposed interface?

ig