aiplatform: provide ability to use Vertex AGENT_IDENTITY

## Is your feature request related to a problem? Please describe.
Vertex Agent Engine introduced agent identity that requires to be explicitly configured, see https://docs.cloud.google.com/agent-builder/agent-engine/agent-identity

Currently, only a ServiceAccount can be specified:
```
type ReasoningEngineSpec struct {
...
// Optional. The service account that the Reasoning Engine artifact runs as.
// It should have "roles/storage.objectViewer" for reading the user project's
// Cloud Storage and "roles/aiplatform.user" for using Vertex extensions. If
// not specified, the Vertex AI Reasoning Engine Service Agent in the project
// will be used.
ServiceAccount *string `protobuf:"bytes,1,opt,name=service_account,json=serviceAccount,proto3,oneof" json:"service_account,omitempty"`
```

## Describe the solution you'd like
Go SDK to do the following:
```
import vertexai
from vertexai import agent_engines
from vertexai import types

client = vertexai.Client(
project=PROJECT_ID,
location=LOCATION,
http_options=dict(api_version="v1beta1")
)
remote_app = client.agent_engines.create(
config={"identity_type": types.IdentityType.AGENT_IDENTITY}
)
```