Adds support for integrating client members via plugins

[skotambkar]

Changes

• Adds support for integrating client members thru Runtime plugins

type Client struct {
    options Options

    \\ client members added via runtime plugins
    cache map[string]string
}

• Adds support to define resolver functions for client members thru Runtime plugins

• Updates existing AddOperationMiddleware helper functions to be methods on client - this helps middlewares have access to client and be able to use client members.