Add decode_at_reference_time method to TokenCoder

* Allows a token expiration timestamp to be compared to a given
  timestamp, instead of the timestamp for the current time.

[#68014448]

Signed-off-by: Eric Malm <emalm@pivotallabs.com>
Bump version to 2.1.0

* Minor version bump for addition of
  TokenCoder#decode_at_reference_time.

[#68014448]

Signed-off-by: Eric Malm <emalm@pivotallabs.com>