Support TransactionOptions.

[stephenplusplus]

Carrying over googleapis/google-cloud-node#2708

Change Overview and Requirements: googleapis/google-cloud-python#4278

This PR:

• Updates the Datastore proto files
• Allows setting the BeginTransaction rpc's TransactionOptions.

Examples:

Global-Level Setting

var transaction = datastore.transaction({ readOnly: true });
transaction.run(function(err) {
  transaction.get(key, function() { /*...*/ });
});

var transaction = datastore.transaction({ id: 'previous-id' });
transaction.run(function(err) {
  transaction.save({ key, data });
  transaction.commit(function(err) {});
});

Method-Level Setting

var transaction = datastore.transaction();
transaction.run({ readOnly: true }, function(err) {
  transaction.get(key, function() { /*...*/ });
});

var transaction = datastore.transaction();
transaction.run({ transactionId: 'previous-id' }, function(err) {
  transaction.save({ key, data });
  transaction.commit(function(err) {});
});

Raw Configuration (method level only)

var options = {
  transactionOptions: {
    readWrite: {
      previousTransaction: 'previous-id'
    }
  }
};

var transaction = datastore.run(options, function(err) {
  transaction.save({ key, data });
  transaction.commit(function(err) {});
});