Add feature to fetch env variables from OS

[abhinavs1920]

PR Description

This PR adds support for fetching environment variables directly from the OS in API Dash. Users can now specify in the environment manager that a variable should be pulled from the system's environment variables instead of being stored in API Dash. The feature enhances security by allowing users to reference sensitive credentials (e.g., API keys, tokens, passwords) without exposing them in the app.

Related Issues

• Closes #600

Checklist

• [x] I have gone through the contributing guide
• [x] I have updated my branch and synced it with project main branch before making this PR
• [x] I am using the latest Flutter stable branch (run flutter upgrade and verify)
• [x] I have run the tests (flutter test) and all tests are passing

Added/updated tests?

• [x] Yes, tests has been added for the files which are modified by me.
• [ ] No, and this is why:

OS on which you have developed and tested the feature?

• [x] Windows
• [ ] macOS
• [ ] Linux

[abhinavs1920]

@animator Please check this, I have added the tests.

[animator]

@animator Please check this, I have added the tests.

sure

[abhinavs1920]

@animator Please check this, I have added the tests.

sure

@animator Can you also please look into this PR also. 😄