When tracing is not configured, potential null pointer dereference when accessing Tracing::Config in filters

*Title*: When tracing is not configured, potential null pointer dereference when accessing Tracing::Config in filters

*Description*:
Without tracing configured, calls to `Tracing::Config::operationName()`, `Tracing::Config::verbose()`, and `Tracing::Config::maxPathTagLength()` via `StreamFilterCallbacks::tracingConfig()` lead to a null pointer dereference.

`HttpConnectionManagerConfig::tracing_config_` is only initialized if tracing config is present, but `ConnectionManagerImpl::ActiveStream` (which implements `Tracing::Config` and forwards some of the calls to `tracing_config_`) calls methods on `tracing_config_` without first performing a null check.

As far as I can tell, there isn't even a way to determine from a filter whether tracing is configured or not.

An easy solution would be to make `StreamFilterCallbacks::tracingConfig()` return a pointer rather than a reference so that the caller knows whether tracing is configured or not.

*Repro steps*:
1. Create an HTTP filter that calls `tracingConfig().verbose()` on a decoder callbacks instance and run it with a configuration that does not have tracing enabled.

*Admin and Stats Output*: N/A
*Config*: N/A
*Logs*: N/A
*Call Stack*: N/A