feat(bigtable): Add update value type test.

[hopha95]

Add new string UTF8 bytes encoding and a deprecate comment to the old encoding. Add test to update value_type of a column family.

[hopha95]

Integration test result:

go test -test.run="TestIntegration_UpdateFamilyValueType" -v \
-it.admin-endpoint="test-bigtableadmin.sandbox.googleapis.com:443" \
-it.project="google.com:cloud-bigtable-dev" \
-it.instance="phhoang-btql-test" \
-it.cluster="phhoang-btql-test-c1"
Note: when using prod, you must first create an instance:
cbt createinstance phhoang-btql-test phhoang-btql-test phhoang-btql-test-c1 us-central1-b 1 SSD
=== RUN   TestIntegration_UpdateFamilyValueType
--- PASS: TestIntegration_UpdateFamilyValueType (6.05s)
PASS
ok      cloud.google.com/go/bigtable    79.286s

[bhshkh]

Please resolve vet failures