gateway: add 429 handling and ErrGatewayRetryAfter

[lidel]

We need a mechanism for signaling to the client when gateway has a cached failure, and when it is ok to retry.
The initial use will be filecoin-saturn/caboose#44 but this will be useful beyond that – in the future, gateway operators will be able to write custom throttling logic, and this error will take care of HTTP semantics.

I propose we:

• reuse HTTP 429 Too Many Requests error code with Retry-After HTTP header
• in go-libipfs/gateway: add special handling of gateway.ErrGatewayRetryAfter in the handler.go#L565-L578, so it produces HTTP 429 response and if ErrGatewayRetryAfter.RetryAfter attribute is set to value >0 set the header on the response.
• specs are already in place https://github.com/ipfs/specs/blob/main/http-gateways/PATH_GATEWAY.md#429-too-many-requests