Add support for testing with WireMock

Hi,

We have some tests that verify our application can connect and interact with DropBox. Ideally, we'd like to be able to run these tests as part of our CI builds without requiring user credentials to be set up and shared.

I tried to use WireMock to capture real data for use in the mocked interactions, but ran into a couple of issues:
1. Although there is a public constructor on DbxClientV2 that accepts a DbxHost object with custom URLs, there wasn't one with the correct combination of parameters that we needed. Our application uses the [`DbxClientV2(DbxRequestConfig, DbxCredential)`](https://dropbox.github.io/dropbox-sdk-java/api-docs/v5.4.3/com/dropbox/core/v2/DbxClientV2.html#%3Cinit%3E(com.dropbox.core.DbxRequestConfig,com.dropbox.core.oauth.DbxCredential)), so I'd have liked to append the `DbxHost` argument to that. Or failing that, call the private all-args constructor, which is essentially what I ended up doing via reflection.
2. Even with a client configured to use a WireMock endpoint at `localhost:{randomPort}`, I got a `URISyntaxException` when trying to connect:
```
java.lang.RuntimeException: URI creation failed, host="localhost:38343", path="2/files/get_metadata": Malformed IPv6 address at index 9: https://[localhost:38343]/2/files/get_metadata
at com.dropbox.core.util.LangUtil.mkAssert(LangUtil.java:15)
at com.dropbox.core.DbxRequestUtil.buildUri(DbxRequestUtil.java:73)
at com.dropbox.core.DbxRequestUtil.startPostRaw(DbxRequestUtil.java:269)
at com.dropbox.core.v2.DbxRawClientV2$1.execute(DbxRawClientV2.java:146)
at com.dropbox.core.v2.DbxRawClientV2.executeRetriable(DbxRawClientV2.java:330)
at com.dropbox.core.v2.DbxRawClientV2.executeRetriableWithRefresh(DbxRawClientV2.java:351)
at com.dropbox.core.v2.DbxRawClientV2.rpcStyle(DbxRawClientV2.java:137)
at com.dropbox.core.v2.files.DbxUserFilesRequests.getMetadata(DbxUserFilesRequests.java:1571)
at com.dropbox.core.v2.files.DbxUserFilesRequests.getMetadata(DbxUserFilesRequests.java:1603)
```
Are there any plans to make the host and port configurable?