Directory Client

[ronenh]

This PR introduces a client/directory package with the following types:

directory.Client: A struct with Reader, Writer, Importer, and Exporter fields.
directory.Config: Configuration for constructing a directory client.

A directory client can be created in a similar way as authorizer clients:

dir, err := directory.New(ctx, client.WithTenantID("..."), client.WithAPIKeyAuth("..."))

Config can be used to create a directory client with customized configuration for each service:

cfg := &directory.Config{
    Reader: &client.Config{Address: "localhost:9292"},
    Writer: &client.Config{Address: "localhost:9293"},
}
dir, err := cfg.Connect(ctx)

In this case, dir.Reader and dir.Writer are initialized with the given configuration. dir.Importer and dir.Exporter are nil.

It's also possible to provide fallback configuration that is used unless overridden:

cfg := &directory.Config{
    Config: &client.Config{Address: "directory.prod.aserto.com:8443", TenantID: "???"},
    Reader: &client.Config{Address: "localhost:9292"},
}

dir, err := cfg.Connect(ctx)

In this case, all services connect to the hosted directory, except reader which connects to localhost.