gateway: dedicated error type for denylists

Right now we don't use proper error types to convey that some content requested by a gateway was blocked. Instead we have this string based checking tied to nopfs

https://github.com/ipfs/boxo/blob/b101ba0858150cd6a34b6cb68eefd28c411bd58c/gateway/errors.go#L211-L216

@MichaelMure recently let me know that:
1. Infura also does some string based error checking for their errors
2. They return 451 when rather than just 410

So it seems to me like this should be bundled into boxo tooling.
1. Have an error type indicating that it's a blocked error (which other tools like nopfs or others can wrap)
2. Given that 451 and 410 both seem plausible either that should be configurable by the error tooling, or at the gateway level
- At the moment I don't know anyone mixing 410s + 451s but that may not always be the case

@MichaelMure let me know if there's any context I missed.

cc @lidel