pass client cert as stream to auth provider

As reported here:
http://svn.haxx.se/dev/archive-2011-06/0287.shtml

Filing an issue so it won't get lost.

The callback for client certs currently has the following format:
svn_error_t* callback(svn_auth_cred_ssl_client_cert_t **cred, void
*baton, const char * realm, svn_boolean_t /*may_save*/, apr_pool_t *pool);

As you can see, the callback expects a path to a file.

That callback should be extended (of course that requires a new versioned
callback) to take a stream instead of a disk file.

The reason I would like this extension is that I can get the client
certificates from the Windows cert store, but there I get the
certificates in memory only. So now I have to save that memory cert
first to a file so I can pass it back in the callback. 

I can't set a target milestone, but I think this should be considered post 1.7