Since the validation of connection blocks is delegated to the communicator
selected by "type", we were not previously doing any validation of the
attribute names in these blocks until running provisioners during apply.
Proper validation here requires us to already have the instance state,
since the final connection info is a merge of values provided in config
with values assigned automatically by the resource. However, we can do
some basic name validation to catch typos during the validation pass, even
though semantic validation and checking for missing attributes will still
wait until the provisioner is instantiated.
This fixes#6582 as much as we reasonably can.
This disables the computed value check for `count` during the validation
pass. This enables partial support for #3888 or #1497: as long as the
value is non-computed during the plan, complex values will work in
counts.
**Notably, this allows data source values to be present in counts!**
The "count" value can be disabled during validation safely because we
can treat it as if any field that uses `count.index` is computed for
validation. We then validate a single instance (as if `count = 1`) just
to make sure all required fields are set.
Martin Atkins