• expose http connection mgr with backwards compatibility
• add HttpClientConnectionManager to SpotifyHttpManager.builder
• add SpotifyHttpManager.Builder.getConnectionManager to handle default

Similar to #375, but providing backwards compatibility to be included in a 8.4.2.

The default usage does not change, however adding support for PoolingHttpClientConnectionManager would look like this:

    var spotifyApi = SpotifyApi.builder()
        .setHttpManager(new SpotifyHttpManager.Builder()
            .setConnectionManager(new PoolingHttpClientConnectionManager())
            .build())
        .setClientId("foo")
        .setClientSecret("bar")
        .build();