Add ServiceTimeout rule

[ConnorKirk]

Issue #, if available:
#3834
Description of changes:

The ServiceTimeout rule emits a warning if the ServiceTimeout property is not specified for a CloudFormation Custom Resource.

• Add new ServiceTimeout Class
• Add test_service_timeout unit test
• Add good/bad fixtures service_timeout.py

Todo

• Fix tests. A few seem to be broken by this new rule
• Double check the ID
• Add the rule to the rules.md?

Notes

• I picked a rule id that seemed available. I wasn't clear if there was a reason for the jump from W045 to W3663. A different ID might be more suitable
• This could be a warning or information level rule. I'm leaning towards warning.
• CloudFormation Custom Resources have multiple possible resourcetypes, either AWS::CloudFormation::CustomResource, a custom resource type, prefixed wit Custom::. I added a small helper function to find all custom resources

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