chore(backend): update wallet address middleware & error handling

[mkurapov]

Changes proposed in this pull request

To finalize the standardization of Open Payments route errors in backend, this PR splits up walletAddressMiddleware into several separate middlewares. One group of them deals with adding the walletAddressUrl onto the ctx (from several resources), while getWalletAddressForSubresource simply polls for the active wallet address by using the ctx.walletAddressUrl itself. Each of the middlewares is then added explicity to the route.

The benefits:

• We now handle all possible Open Payments API errors properly in the openPaymentsServerErrorMiddleware (i.e. we don't use ctx.throw at all in other places)
• walletAddressUrl is added to the ctx as a middleware specific to the route route, meaning we can remove all-encompassing logic in walletAddressMiddleware. Likewise, we don't need to actually fetch the wallet address before doing token introspection and http sig verification

Other changes:

• When wallet address is requested, return the Open Payments one by default.
• SPSP payment pointer can only be fetched when setting accept header to include 'application/spsp4+json' explicitly

Context

Fixes #1905 for backend

Checklist

• [x] Related issues linked using fixes #number
• [x] Tests added/updated
• [ ] Documentation added
• [ ] Make sure that all checks pass
• [ ] Bruno collection updated

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[BlairCurrey]

• walletAddressUrl is added to the ctx as a middleware specific to the route route, meaning we can remove all-encompassing logic in walletAddressMiddleware. Likewise, we don't need to actually fetch the wallet address before doing token introspection and http sig verification

This is much better. Low coupling/high cohesion. 👍

[mkurapov]

Going to close and re-open to see if I can get past the stuck actions (I think affected by their degradation here)