Add type coercion from Map to ProxySslConfig

Adds a new type coercion that converts the `Map` data generated from a YAML config into a `ProxySslConfig` object. Expects data like this:

```JS
- serviceType: brooklyn.entity.proxy.nginx.NginxController
  id: nginx
  brooklyn.config:
    ssl:
      certificateSourceUrl: "classpath://server.crt"
      keySourceUrl: "classpath://server.key"
```
Any `ConfigKey` definition with the `ProxySslConfig` type (for example in
the `LoadBalancer` interface) causes the class static initialisers to be
called.