feat: adding variables support

[shashitnak]

Summary:
Briefly describe the changes made in this PR.

Issue Reference(s):
Fixes #... (Replace "..." with the issue number)

Build & Testing:

• [x] I ran cargo test successfully.
• [x] I have run ./lint.sh --mode=fix to fix all linting issues raised by ./lint.sh --mode=check.

Checklist:

• [x] I have added relevant unit & integration tests.
• [ ] I have updated the documentation accordingly.
• [ ] I have performed a self-review of my code.
• [x] PR follows the naming convention of <type>(<optional scope>): <title>

[github-actions[bot]]

Action required: PR inactive for 5 days. Status update or closure in 10 days.

[meskill]

In favour of https://github.com/tailcallhq/tailcall/pull/2402