Add `/healthz` as a forbidden path during wallet address creation

[mkurapov]

Context

Just like the paths /incoming-payments /outgoing-payments and /quotes, we shouldn't allow a wallet address with a /healthz path to be created via the backend createWalletAddress API, as we use this endpoint as a healthcheck for the Open Payments API.

[devcer]

@mkurapov, I would leave this to other newbies. But I hope this is only about adding the /healthz path to Forbidden paths here? -> https://github.com/interledger/rafiki/blob/main/packages/backend/src/open_payments/wallet_address/service.ts#L138

[kpeale]

@devcer, I do not understand what the issue is about?

[mkurapov]

Assigning @elena-petridou