Setting up secret scanning GitHub Action

[mliao95]

Description of changes:

This PR adds a new GitHub Action for scanning each PR for strings that match AWS related secret regexes. We can expand this to include other regex patterns further down the line. Here are the blocked secret regexes that come with git-secrets --register-aws

secrets.patterns (A3T[A-Z0-9]|AKIA|AGPA|AIDA|AROA|AIPA|ANPA|ANVA|ASIA)[A-Z0-9]{16}
secrets.patterns ("|')?(AWS|aws|Aws)?_?(SECRET|secret|Secret)?_?(ACCESS|access|Access)?_?(KEY|key|Key)("|')?\s*(:|=>|=)\s*("|')?[A-Za-z0-9/\+=]{40}("|')?
secrets.patterns ("|')?(AWS|aws|Aws)?_?(ACCOUNT|account|Account)_?(ID|id|Id)?("|')?\s*(:|=>|=)\s*("|')?[0-9]{4}\-?[0-9]{4}\-?[0-9]{4}("|')?

See example of this action blocking a PR here: #2

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