fixes #590

Until now the thread querying the application endpoint can be blocked
for a very long time when the application is inresponsive. With this
commit two new properties `spring.boot.admin.monitor.read-timeout`
(default: 2s) and `spring.boot.admin.monitor.connect-timeout` (default:
5s) are added.

closes #462

With this change it is possible to include credentials in the instance
metadata which will then be used to access the client endpoints using
HTTP Basic authorization.

closes #359