fix(spanner): add SpannerAsyncClient import to spanner_v1 package

[harshachinta]

The auto-generated snippets in SpannerAsyncClient documentation mentions the import as follows,

from google.cloud import spanner_v1

async def sample_batch_create_sessions():
    # Create a client
    client = spanner_v1.SpannerAsyncClient()

The above sample gives the following error

AttributeError: module 'google.cloud.spanner_v1' has no attribute 'SpannerAsyncClient'. Did you mean: 'SpannerClient'

This PR adds changes to fix this issue.