Add a way to automatically redact the middle of high-entropy strings

Because the generated reports contain sensitive information, care should be used when sharing them.

The tool would still be useful in assessing differences between environments if the middle portion of key, passwords, etc. were blocked out by default.

Start by locating the report-generation and environment-variable comparison entry points in env-diff; no file or test is named in the issue. Define how high-entropy values, including keys and passwords, should be identified and how their middle portions should be redacted. Done means reports retain useful environment differences without exposing sensitive middle content.

Written by the indexing model from the issue text.