Add pairing and cloud migration APIs to SDK

[sharma-tapas]

Is this a BUG REPORT or FEATURE REQUEST?: FEATURE REQUEST

What happened: Currently, the SDK does not have support for

1. Cluster Pairing
2. Volume Cloud Migration This PR should be adding the following API's

- Cluster Pairing

• [ ] Create : Create pair with a remote cluster
• [ ] Process : handles an incoming pair request from a remote cluster
• [ ] Get : returns pair information for a cluster
• [ ] Enumerate : returns list of cluster pairs
• [ ] Delete : Delete a cluster pairing
• [ ] GetPairToken : gets the authentication token for this cluster

- Volume Cloud Migration

• [ ] Start : starts a migrate operation
• [ ] Cancel : cancels/stops a migrate operation
• [ ] Status : status of a migrate operation

We will be creating a new service for the cluster pairing called OpenStorageClusterPair and for volume cloud migration as OpenStorageVolumeCloudMigrate which will house the above.