Add: `res.getSize()` method with Postman compatibility for response s…

[pooja-bruno]

Description

This PR adds a new res.getSize() method to Bruno's response object, providing compatibility with Postman's pm.response.size() functionality. The implementation allows users to measure response sizes in byte.

Contribution Checklist:

• [x] The pull request only addresses one issue or adds one feature.
• [x] The pull request does not introduce any breaking changes
• [ ] I have added screenshots or gifs to help explain the change if applicable.
• [x] I have read the contribution guidelines.
• [x] Create an issue and link to the pull request.

Note: Keeping the PR small and focused helps make it easier to review and merge. If you have multiple changes you want to make, please consider submitting them as separate pull requests.

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