add support for `Envoy Timer API`

[yskopets]

Context

• Envoy Timer API consist of
  • proxy_set_tick_period
  • proxy_on_tick

Blockers

• at the moment, proxy-wasm only supports setting 1 timer per "root context" (equivalent of Envoy Extension Factory singleton)
• it would be confusing to expose such API to HTTP Filter and Network Filter
• we could probably expose such API to AccessLogger, but it's not yet operational (https://github.com/envoyproxy/envoy-wasm/issues/487)