Pccs - sync cache get should do network fetch

[ameba23]

Currently the PCCS implementation will avoid doing a network fetch during the sync cache getter. This is because it is called from the rustls certificate verifier trait method which is synchronous, and so should not block over a network call when run in an async runtime.

In practice this is quite an issue for usability - see #58

This PR is one possible solution to this problem. There is another solution i am also considering, and both have tradeoffs. Ideally both solutions should be reviewed together so we can choose the best.

We take advantage of the latest dcap-qvl CollateralClient api which lets us pass in a custom http client. This allows us to use ureq (a synchronous http client) for collateral fetches whilst still using dcap-qvl's collateral fetching code (which by default uses reqwest).

Here, we fetch collateral if it is not available in the cache, rather than bailing and starting a spawned task to fetch in the background.