Support proto in AsyncZipkinSpanHandler

[codefromthecrypt]

Right now, it is required to use JSON when using brave's direct encoder.

Caused by: java.lang.IllegalArgumentException: Encoder doesn't match Sender: JSON PROTO3
	at zipkin2.reporter.AsyncReporter$Builder.build(AsyncReporter.java:201)
	at zipkin2.reporter.brave.AsyncZipkinSpanHandler.<init>(AsyncZipkinSpanHandler.java:152)
	at zipkin2.reporter.brave.AsyncZipkinSpanHandler$Builder.build(AsyncZipkinSpanHandler.java:147)
	at zipkin2.server.internal.brave.ZipkinSelfTracingConfiguration.reporter(ZipkinSelfTracingConfiguration.java:64)
	at java.base/jdk.internal.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
	at java.base/jdk.internal.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62)
	at java.base/jdk.internal.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
	at java.base/java.lang.reflect.Method.invoke(Method.java:564)
	at org.springframework.beans.factory.support.SimpleInstantiationStrategy.instantiate(SimpleInstantiationStrategy.java:154)
	... 140 more