Expose additional BoxAPIRequest/BoxAPIConnection members for request intercept use case

In a Box SDK integration, I implemented a `com.box.sdk.RequestInterceptor` that uses its own HTTP client to send the requests over the network. Doing that, I noticed that in order to support this scenario, certain members should, in my opinion, be exposed to clients outside the SDK:
- `com.box.sdk.BoxAPIRequest.getHeaders()` should be public so that the corresponding content is accessible
- `com.box.sdk.BoxAPIRequest.shouldAuthenticate` should be exposed via a getter to distinguish the two cases that it indicates

In order to hold a read lock on the access token during the request (as `com.box.sdk.BoxAPIRequest.trySend(ProgressListener)` does), it would be necessary to also expose
- `com.box.sdk.BoxAPIConnection.lockAccessToken()` and
- `com.box.sdk.BoxAPIConnection.unlockAccessToken()`.

The access token can be obtained with the (already public) method `com.box.sdk.BoxAPIConnection.getAccessToken()`, but in this case there is no lock and the access token could be invalidated before the HTTP request is sent (or while it is being sent).