Introduce FetchDetails for Fetch Instance/Binding APIs

[NickyMateev]

Introduce FetchDetails for Fetch Instance/Binding APIs

This pull request introduces FetchDetails for the OSB Fetch Service Instance & Fetch Service Binding requests.
As specified in the spec:

• Fetch Service Instance: https://github.com/openservicebrokerapi/servicebroker/blob/master/spec.md#parameters-3
• Fetch Service Binding: https://github.com/openservicebrokerapi/servicebroker/blob/master/spec.md#parameters-7

Both handlers should be able to accept service_id and plan_id parameters. Currently the brokerapi library doesn't expose that to library clients, which in some situations is vital information (even if OSB spec doesn't mandate these properties).

With this PR the intention is to allow library clients to have access to these request details.

Most notably the GetInstance handler now takes an additional argument FetchDetails like so:

//   GET /v2/service_instances/{instance_id}
GetInstance(ctx context.Context, instanceID string, details FetchDetails) (GetInstanceDetailsSpec, error)

Similarly with the GetBinding handler:

//   GET /v2/service_instances/{instance_id}/service_bindings/{binding_id}
GetBinding(ctx context.Context, instanceID, bindingID string, details FetchDetails) (GetBindingSpec, error)

Tests for the new handler arguments are included as well.

[cf-gitbot]

We have created an issue in Pivotal Tracker to manage this:

https://www.pivotaltracker.com/story/show/176334751

The labels on this github issue will be updated when the story is started.

[mweibel]

What's the status of this? Would be great to have this merged in 👍

[FelisiaM]

Thanks for the contribution @NickyMateev!

I have created #147 to do some tidying up and will close this PR in favour of that one.