Ability to locally cache responses from CCDB header-only requests

[sawenzel]

This provides the ability to store responses from CcdbApi::retrieveHeaders calls to the disc, from where it can be picked up before trying to do more requests to the server.

Responses are serialized into json format. Caching is timestamp sensitive.

Caching is enabled when ALICEO2_CCDB_LOCALCACHE is set, just like when retrieving CCDB objects.

Fixes https://its.cern.ch/jira/browse/O2-4691

[github-actions]

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