Add support for only-if-cached HEAD behavior

[2color]

Background

When testing providers coming from the IPNI, it's useful to be able to check if they have a block without requesting the bytes.

In the IPFS HTTP Gateway Spec this is specified as follows:

1.2.1 only-if-cached HEAD behavior
HTTP client can send HEAD request with Cache-Control: only-if-cached to disable IPFS data transfer and inexpensively probe if the gateway has the data cached.

Implementation MUST ensure that handling only-if-cached HEAD response is fast and does not generate any additional I/O such as IPFS data transfer. This allows light clients to probe and prioritize gateways which already have the data.

[rvagg]

👍 that would be pretty easy to implement I expect