feat: Hide OS API key by allowing proxy endpoint

[DafyddLlyr]

What does this PR do?

• Support the use of a proxy via the osProxyEndpoint property
• This means that we do not need to expose the OS API key when making requests

How is this achieved?

Before making any requests to the OS, we get the URL from ordnanceSurvey.getServiceURL().

If the user supplied an OS API key, we'll query the OS API directly and append their key. This is the exact same functionality as currently exists.

If the user supplies an osProxyEndpoint value, we'll construct the correct service URL, without an API key. It is the end user's responsibility to host and maintain a suitable service to proxy requests (with the correct authentication) to the OS API. Ensuring proper access to the proxy endpoint can be done through CORS/CORP on the proxy. The documentation in this repository outlines how this can be achieved.

Testing

In order to test this, you can run the dev server locally and add the osProxyEndpoint="https://api.planx.dev/proxy/ordnance-survey" property to the my-map or address-autocomplete components. This will also work on the "Deploy Preview" link generated by the PR.

A number of unit test and service / integration tests have also been added to cover the new functionality, but there's plenty more that could be added to cover all scenarios. I think I've captured the main journeys here but please let me know if you think we need more coverage anywhere.

Documentation

Pitsby doesn't allow markup or any rich text (even line breaks) in it's descriptions, so linking back to a richer markdown page seemed helpful here.

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[jessicamcinchak]

Looking really solid 🎉 I want to come back with fresher eyes at least once more, but here's initial comments/questions. Appreciate the docs, examples and tests here - makes it much easier to review!