Decoding keys from FSR

Get the metadata from the FSR. LHCbIntegrationTests!64 (diffs)

See LHCb!4598 (merged)

@graven @dfazzini @eleckste @nskidmor

Points from the discussion:

• Use a version number so the reading knows exactly what keys can be expected to be in the FSR
  • There is a well defined schema for what to expect in the FSR rather then a recursive search for possible keys
  • Writing the FSR should always use the latest version of the schema
• Makes it backwards and forwards compatible -- the current version without any explicit version number can be recognized by its lack of version number, i.e. the version of an FSR is the value of the key version or, if that key is not present, 0.

Feel free to add information etc here