payment-circle: polish responses and code

[marcelosalloum]

PR Checklist

PR Structure

• This PR has reasonably narrow scope (if not, break it down into smaller PRs).
• This PR avoids mixing refactoring changes with feature changes (split into two PRs
  otherwise).
• This PR's title starts with name of package that is most changed in the PR, ex.
  paymentservice.stellar, or all or doc if the changes are broad or impact many
  packages.

Thoroughness

• This PR adds tests for the most critical parts of the new functionality or fixes.

What

Code polishes and small fixes. Changes details:

• Add CircleWallet.defaultCapabilities() and CircleWallet.merchantAccountCapabilities() to avoid hardcoding the capabilities every time.
• Update Account default balances and unsettledBalances values to null, so the JSON response works better when we don't have the balances. Otherwise, returning an empty list passes the impression the balances are empty when that might not be the case.
• Fix paginated responses cursors.
• Updated docs on how to get the bank deposit instructions.
• Solve warnings
• Update CirclePaymentServiceTest to reduce code repetition and standardize method names.
• Add all Circle asset names to the CircleAsset class, so we don't need to hardcode the string names.