Add script to audit and generate missing service configurations

[brendanhay]

Adds simplistic audit functionality from generate.nix that was removed in #639. Additionally, it now writes example skeleton configurations and annexes to the output directory specified by the singular script argument.

Example:

% generate-configs tmp
...
Missing wafv2
Missing wellarchitected
Missing wisdom
Missing worklink
Missing workmailmessageflow
Missing 141 service configurations.
Wrote examples to tmp/configs and tmp/annexes.
Found 281 models in /nix/store/bvn31qxjpffkaa5misrb8y0kxphhy0gw-source/botocore/data.
Found 140 service configurations in config/services.
Done.

The reason for also generating an empty annex file is illustrated by testing with the currently unsupported textract service (#509) - it requires metadata.serviceAbbreviation to be specified since it's missing from the definition and this is used to infer the root module namespace. At some point in the future I'll get around to merging the configurations to remove the need for annex at all.

[endgame]

Is nix-shell smart enough to resolve this relative to the script, and not to the working directory?

[brendanhay]

It resolves relative to the script location - testing that out did reveal that cabal needs to be added to the buildInputs, though.