feat: add support for using the emulator programatically

[larkee]

Currently, we do not support setting the emulator host outside of the env flag SPANNER_EMULATOR_HOST.

This PR allows users to set the emulator host by using the api_endpoint in client_options with google.auth.credentials.AnonymousCredentials

e.g.

from google.auth.credentials import AnonymousCredentials
from google.cloud import spanner

cred = AnonymousCredentials()
client_options = {'api_endpoint': '0.0.0.0:9010'}
client = spanner.Client(
    credentials=cred,
    client_options=client_options,
    project='emulator-project'
)

[snehashah16]

for other libraries, we may have done the opposite.
If the emulator_env_variable is set, we hard-code anonymous/null credentials for the emulator.

[larkee]

I have updated it to set AnonymousCredentials when the env var is set.

[hengfengli]

LGTM. A few comments.

[hengfengli]

LGTM.