Previously resources were assumed to always support the full set of
create, read, update and delete operations, and Terraform's resource
management lifecycle.
Data sources introduce a new kind of resource that only supports the
"read" operation. To support this, a new "Mode" field is added to
the Resource concept within the config layer, which can be set to
ManagedResourceMode (to indicate the only mode previously possible) or
DataResourceMode (to indicate that only "read" is supported).
To support both managed and data resources in the tests, the
stringification of resources in config_string.go is adjusted slightly
to use the Id() method rather than the unusual type[name] serialization
from before, causing a simple mechanical adjustment to the loader tests'
expected result strings.
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