cdk - BYO network transport PR

[vnprc]

Hashpool currently uses a fork of cdk hosted at github.com/vnprc/cdk branch name hashpool. Extract this code diff into a consistent change set and submit it to cdk as a PR.

• rebase the branch to bring it up to the most recent cdk release
• in the PR explain the features this adds
  • ability to generate ecash tokens without relying on the built in HTTP network transport
  • store premint secrets in the wallet while awaiting mint response
• unit tests
• conform to cdk coding style and build requirements

[vnprc]

cashubtc/cdk#605