Created a ./lib folder for reusable modules.

[v0lkan]

Description

This PR creates a ./lib folder for the code that is independent of the project
and may be either converted to a standalone repository, or can be imported and
used by other projects without directly relying on the VSecM components.

Modules ported to ./lib:

• ./lib/template: Generates pattern-based random strings.
• ./lib/backoff: A generic backoff/retry library.
• ./lib/system: Contains a method that exits the app gracefully when it
  receives a SIGINT or SIGKILL.

Test Policy Compliance

• I have added or updated unit tests for my changes.
• I have included integration tests where applicable.
• All new and existing tests pass successfully.

Code Quality

• I have followed the coding standards for this project.
• I have performed a self-review of my code.
• My code is well-commented, particularly in areas that may be difficult
  to understand.

Documentation

• I have made corresponding changes to the documentation (if applicable).
• I have updated any relevant READMEs or wiki pages.

Checklist

Before you submit this PR, please make sure:

• You have read the contributing guidelines and
  especially the test policy.
• You have thoroughly tested your changes.
• You have followed all the contributing guidelines for this project.
• You understand and agree that your contributions will be publicly available
  under the project's license.

By submitting this pull request, you confirm that my contribution is made under
the terms of the project's license and that you have the authority to grant
these rights.

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Thank you for your contribution to VMware Secrets Manager
🐢⚡️!

[vmwclabot]

@v0lkan, you must sign every commit in this pull request acknowledging our Developer Certificate of Origin before your changes are merged. This can be done by adding Signed-off-by: John Doe <[email protected]> to the last line of each Git commit message. The e-mail address used to sign must match the e-mail address of the Git author. Click here to view the Developer Certificate of Origin agreement.

• Commit 8ef8945 must be signed by [email protected]