Provide middlewares for koa

[bzums]

Currently we provide middlewares for expressjs to:

1. conveniently extract and validate an oauth2 access token from a response (handleOAuthRequestMiddleware, see https://github.com/zalando-incubator/authmosphere/blob/master/src/express-tooling.ts#L85)
2. conveniently secure an endpoint via requiring an access token to have certain scopes (requireScopesMiddleware, see https://github.com/zalando-incubator/authmosphere/blob/master/src/express-tooling.ts#L40)

For both middlewares it would be nice to have a pendant for koajs.

The requirements for handleOAuthRequestMiddleware are:

• accepts parameter of type MiddlewareOptions
• extracts token from Authorization header of a request
• validates this token via calling getTokenInfo
• makes this token available for later use in the middleware chain

The requirements for requireScopesMiddleware are:

• accepts a parameter of type string[] which defines the scopes necessary for calling an endpoint
• gets the token extracted by handleOAuthRequestMiddleware
• checks whether the given token has all necessary scopes