refactor OSS Trace proto into Trace metadata and a TraceContainer.

[copybara-service[bot]]

refactor OSS Trace proto into Trace metadata and a TraceContainer.

This CL introduces a OSS TraceContainer type that is very similar to the internal TraceEventsContainer. End goal is to either subsume TraceEventsContainer into TraceContainer, or have it inherit most of its functionality from TraceContainer. (We cannot do this until the OSS Trace proto is fully decoupled from trace events though.)

OSS TraceContainer inherits from abstract AnyTraceContainer, which provides the arena allocator, and TraceEvent associatedtype requirement.