Add support for updating function code in place

[lukehoban]

Remove ForceNew from the source inputs on function resources, and add support for updating the function in place from a new source bucket+object.

This is similar to #1666, but includes test coverage and removes ForceNew from both source_archive_bucket and source_archive_object.

Tests:

$ /usr/local/bin/go test -v -timeout 1h github.com/terraform-providers/terraform-provider-google/google -run ^TestAccCloudFunctionsFunction
=== RUN   TestAccCloudFunctionsFunction_basic
=== RUN   TestAccCloudFunctionsFunction_update
=== RUN   TestAccCloudFunctionsFunction_pubsub
=== RUN   TestAccCloudFunctionsFunction_bucket
--- PASS: TestAccCloudFunctionsFunction_basic (34.82s)
--- PASS: TestAccCloudFunctionsFunction_pubsub (41.58s)
--- PASS: TestAccCloudFunctionsFunction_update (60.10s)
--- PASS: TestAccCloudFunctionsFunction_bucket (60.19s)
PASS
ok      github.com/terraform-providers/terraform-provider-google/google 60.233s

[lukehoban]

/cc @dustinblackman @paddycarver

[danawillow]

Looks good, thanks @lukehoban!

[ghost]

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