Add function to test for existence for webservice

[revodavid]

For CI/CD setups, it a standard pattern would be:

• If a webservice doesn't exist, deploy it with deploy_model
• If the model has already been deployed and the webservice already exists, update it with update_aci_webservice

However, there is no function to detect the existence of a webservice. The workaround is messy:

try({
    existing_service <- get_webservice(ws, 'model_name')
})

It would be easier to write CI/CD flows to update webservices if either:

• get_webservice returned NULL if the named webservice is not found instead of throwing an error, or:
• a new function to check for the existence of a webservice were added

[mx-iao]

First option makes sense - we'll look into catching the error and returning NULL in get_webservice