[lambda][flare] Obfuscate secrets

[nhulston]

What and why?

For more context on what Lambda Flare is, please read the description in this PR: #924

We want to obfuscate secrets (passwords, API keys, and other unknown environment variables) in Lambda function configuration so customers don't send private information to Datadog support. Any non-Datadog environment variables should also be obfuscated.

How?

1. Create a whitelist of 'allowed' environment variables to not be obfuscated. These are stored in a set in the constant NON_OBFUSCATED_ENV_VARS for O(1) lookup.
2. If an environment variable is not in this set, we will pass it to the getObfuscation function which returns an obfuscated version of the string.
3. Short strings (<12 characters) will be entirely obfuscated and replaced with *'s. Longer strings will preserve the first 2 and last 4 characters of the string, but obfuscate everything in between.
4. Numbers and boolean values are unlikely to be secrets, so they can be skipped and returned as-is
5. Obfuscate the function config before logging to console, saving the file, and sending to Datadog support

Review checklist

• Feature or bugfix MUST have appropriate tests (unit, integration)

[datadog-datadog-prod-us1]

Datadog Report

Branch report: nicholas.hulston/serverless-flare/obfuscate-secret-values
Commit report: 51cf701

✅ datadog-ci-tests: 0 Failed, 0 New Flaky, 837 Passed, 0 Skipped, 46.81s Wall Time