- Update to v0.2.1 of corim-store that implements MaxExpiry, which is
  used when CoRIMs do not specify validity, and serves as a maximum
  limit on expiry when they do.
- In order allow configuration of MaxExpiry, refactor how configuration
  for CoSERV is handled inside VTS:
  - Create CoservContext which contains configuration for CoSERVService
    prior to its creation, and also contains the signer (which is not
    part of the service, but is related to it).
  - Configuration for the context is taken from "coserv" section of
    config files (the same as the front-end: all coserv-related
    configuration is now in the same place in the config tree).
    Configuration for the signer is moved to "signer" sub-section under
    it.
- FallbackAuthority for the service is now configured based on the
  signer's public key, rather than being hard-coded to a nonsense value.

Signed-off-by: Sergei Trofimov <sergei.trofimov@arm.com>