Allow creating property file auth providers from a Buffer

[jnbdz]

The property file authentication/authorization providers can only be created from a file system path, and the provider reads that path itself. As described in #728, this prevents loading the content from anywhere else — the classpath inside a fat jar, a remote location — or preprocessing it before parsing (Apache Camel wants to resolve environment variable references in the file, e.g. user.admin=${ADMIN_PASSWORD}).

This adds create(Vertx, Buffer) overloads to both PropertyFileAuthentication and PropertyFileAuthorization, taking the property file content directly:

vertx.fileSystem().readFile("auth.properties")
  .map(buffer -> PropertyFileAuthentication.create(vertx, buffer))

The issue suggested InputStream, but a Buffer felt more idiomatic for Vert.x (@VertxGen-friendly, non-blocking, and it is what FileSystem.readFile returns) while solving the same problem: the caller stays in control of how the content is loaded. The existing path-based overloads are unchanged; parsing is shared between both paths. For the buffer variant, the authorization provider id (previously the path) is the constant properties.

Includes tests and documentation.

Fixes #728