ArmVirtPkg: ArmCcaRsiLib: Add an interface to get an attestation token

A CCA attestation token is a collection of claims about the state of a
Realm and of the CCA platform on which the Realm is running.
A CCA attestation token consists of two parts:
  * Realm token - Contains attributes of the Realm, including:
    # Realm Initial Measurement
    # Realm Extensible Measurements
  * CCA platform token - Contains attributes of the CCA platform
    on which the Realm is running, including:
    # CCA platform identity
    # CCA platform life cycle state
    # CCA platform software component measurements

The CCA attestation token is used by a verification service to validate
these claims.

The Realm Service Interface defines the following interfaces to retrieve
an attestation token from the Realm Management Monitor (RMM).
  - RSI_ATTESTATION_TOKEN_INIT
  - RSI_ATTESTATION_TOKEN_CONTINUE

Therefore, update the ArmCcaRsiLib to add an interface to get an
attestation token from the RMM.

Signed-off-by: Sami Mujawar <sami.mujawar@arm.com>