Run mypy in Key Vault CI

[chlowell]

Fix current errors and add packages to mypy_hard_failure_packages.py:

• azure-keyvault-administration (Run mypy in azure-keyvault-administration CI #19246)
• azure-keyvault-certificates(Run mypy in azure-keyvault-certificates CI #20525)
• azure-keyvault-keys(Run mypy in azure-keyvault-keys CI #20545)
• azure-keyvault-secrets(Run mypy in azure-keyvault-secrets CI #20507)

[mccoyp]

For reference: to run mypy for a package (e.g. azure-keyvault-certificates), run
mypy --exclude _generated --ignore-missing azure
from the top of the package -- in this case, from azure-sdk-for-python/sdk/keyvault/azure-keyvault-certificates

[mccoyp]

Steps to resolve:

For each package:

1. Run mypy check
2. Resolve warnings
3. Once there are no warnings, add package to hard failures file
4. Include a mypy.ini at the package root that tells mypy to ignore generated files. For example:

[mypy]
python_version = 3.6
warn_unused_configs = True
ignore_missing_imports = True

[mypy-azure.keyvault.*._generated.*]
ignore_errors = True