execution context: refactored the impl to use marco to enable the execution context

[wbpcode]

Commit Message: execution context: refactored the impl to use marco to enable the execution context
Additional Description:

The execution context is a feature that was contributed by the @wu-bin from google. But it only makes sense in very limited scenarios.
After a discussion with @wu-bin , we decided to refactor it to make it more 'independent'.

This PR refactored the implementation to make the custom abstraction ExecutionContext only be referenced in limited positions of Envoy core code base. More specifically, this PR:

1. move all ExecutionContext related code to single header file envoy/common/execution_conetxt.h.
2. No any directly dependency to the ExecutionContext at the Envoy core code base except the source/common/common/scope_tracker.h.
3. compiled the ExecutionContext out by default.
4. removed restart feature flag envoy.restart_features.enable_execution_context.
    
   Risk Level: low.
   Testing: unit.
   Docs Changes: n/a.
   Release Notes: added.
   Platform Specific Features: n/a.

[repokitteh-read-only]

CC @envoyproxy/runtime-guard-changes: FYI only for changes made to (source/common/runtime/runtime_features.cc).

🐱

Caused by: #36277 was opened by wbpcode.

see: more, trace.