Add some tooling for multi-person reviews

While most PRs just need a single technical review, PRs adding science need to also have portability and scientific reviews. To get around GitHub only having a single setting for required reviews it would be good to add another system to work around this on a per-PR basis.

There already exist [third party actions to do this](https://github.com/EasyDynamics/pr-review-threshold), but we cannot use them due to the Met Office security policies. Instead we should make a simple action to reimplement them.

Start by reviewing the linked pr-review-threshold action and GitHub's single required-review setting, then compare them with the repository's security-policy constraints. Done means a per-PR mechanism enforces separate portability and scientific reviews without relying on an external third-party action.

Written by the indexing model from the issue text.