Make admin server RESTful only

[andrewemeryanz]

sysl-go currently supports a RESTful admin server by way of the Manager interface:

type Manager interface {
	AdminServerConfig() *config.CommonHTTPServerConfig
}

The GrpcManager interface also defines a server configuration but it isn't supported:

type GrpcManager interface {
	GrpcAdminServerConfig() *config.CommonServerConfig
}

The admin server should be only made available as a RESTful service. To support this, the admin server configuration should be removed from Manager and GrpcManager interfaces in favour of a standalone configuration avenue.