Import jsr305 and use it to mark @Nullable stuff.

This library is a provided dependency and is not necessary to be included
by OkHttp users.

This currently covers the okhttp and okhttp-logging-interceptor modules
only. It does not cover the 'internal' package in OkHttp, or its test cases.
We can add those as needed.

okhttp-logging-interceptor/src/main/java/okhttp3/logging/package-info.java

@@ -0,0 +1,3 @@
+/** An OkHttp interceptor which logs HTTP request and response data. */
+@javax.annotation.ParametersAreNonnullByDefault
+package okhttp3.logging;