fix: Prototype-polluting assignment

[odaysec]

Ticket 🎟️#3705

To fix the prototype pollution vulnerability, we should ensure that the keys used in the combineOverrides method cannot be used to modify the Object.prototype. One effective way to achieve this is by using a Map object instead of a plain object for combinedOverrides. This will prevent any prototype pollution since Map does not have the same prototype properties as plain objects.

Quality Assurance

• [x] If a new adapter was made, or an existing one was modified so that its environment variables have changed, update the relevant infra-k8s configuration file.
• [ ] If a new adapter was made, or an existing one was modified so that its environment variables have changed, update the relevant adapter-secrets configuration file or update the soak testing blacklist.
• [ ] If a new adapter was made, or a new endpoint was added, update the test-payload.json file with relevant requests.
• [x] The branch naming follows git flow (feature/x, chore/x, release/x, hotfix/x, fix/x) or is created from Jira.
• [ ] This is related to a maximum of one Jira story or GitHub issue.
• [ ] Types are safe (avoid TypeScript/TSLint features like any and disable, instead use more specific types).
• [x] All code changes have 100% unit and integration test coverage. If testing is not applicable or too difficult to justify doing, the reasoning should be documented explicitly in the PR.

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[mxiao-cll]

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