feat: add partition field to model

[jannyHou]

connect to https://github.com/strongloop/loopback-connector-cloudant/issues/217

• You can specify a partition key field in model definition like

  customers = db.define('customers', {
  userId: {type: Number, id: true},
  // partition key field
  countryCode: {type: String, isPartitionKey: true},
  name: String,
  zipCode: Number,
  });

This PR adds the name of partition key field to couchdb/cloudant's model object. The corresponding cloudant PR is https://github.com/strongloop/loopback-connector-cloudant/pull/230

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