2.1.0

• Added the means to work with custom tokens and ID tokens by using
  kreait/firebase-tokens. See
  Authentication: Working with Tokens
  for usage instructions.
• Replaced the implementation of Database Secret based custom tokens (in the V2 namespace)
  with a solution based on lcobucci/jwt instead of the
  abandoned firebase/token-generator.

These features are also available in a standalone library: kreait/firebase-tokens