Detect mismatching AWS AZ and subnet at plan time

[jleclanche]

Reopening #645 which was closed for no good reason other than being inactive.

How to reproduce:

Create an aws_subnet with the availability zone us-east-1b
Create an aws_instance, specify the subnet and specify the availability zone as something other than us-east-1b (eg. us-east-1c)
The instance creation will error:

* aws_instance.test_01: Error launching source instance: InvalidParameterValue: Value (us-east-1c) for parameter availabilityZone is invalid. Subnet 'subnet-aff9fe92' is in the availability zone us-east-1b
status code: 400

This can be caught at plan time; ideally by checking that the AZ and subnet AZ match, or by making AZ and subnet mutually exclusive.

[github-actions]

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[jleclanche]

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