feat: easier testing of a specific specification/IPIP

## Overview

Some of our tests mention/link to a specification, e.g. https://github.com/ipfs/gateway-conformance/blob/e782d03f9b637e32bb908558725e501523046fae/tests/path_gateway_tar_test.go#L79, but not all of them.

## Goal

We should make it easier for folks who are reading specs, to run conformance tests for that specific test, and potentialy even target specific sections of those specs.

### Examples

Trying to test https://specs.ipfs.tech/http-gateways/trustless-gateway could be as easy as:

```
gateway-conformance test <...> --specs http-gateways/trustless-gateway
```

You could target a subsection of that test (i.e. https://specs.ipfs.tech/http-gateways/trustless-gateway/#dag-scope-request-query-parameter) with something like:

```
gateway-conformance test <...> --specs http-gateways/trustless-gateway/dag-scope-request-query-parameter
```

## Concerns and other thoughts

Tight coupling these like this will cause some pain, but it will also keep our specifications and conformance tests accurate and up to date.

## Related

Potentially related? https://github.com/ipfs/gateway-conformance/issues/120

Similar to https://github.com/ipfs/gateway-conformance/issues/86, but for keeping conformance tests closer to specifications.