Add /account_balance endpoint

[FabijanC]

Usage related changes

• Closes API account_balance has error #393
• Adds the same /account_balance that existed in Devnet-py, but with added token specifier:
  • GET /account_balance?address=<ADDRESS>&[unit=<WEI|FRI>]
• Remove /fee_token - nobody's asked for it, it seems to be a remnant from times when ERC20 contracts were not deployed to the same addresses as on mainnet/testnet.
• Improve GeneralError message

Development related changes

• Refactor BackgroundDevnet::get_balance to rely on the new endpoint.
• Refactor existing minting tests:
  • Add /account_balance testing
  • Simplify existing tests

Checklist:

• Applied formatting - ./scripts/format.sh
• No linter errors - ./scripts/clippy_check.sh
• No unused dependencies - ./scripts/check_unused_deps.sh
• Performed code self-review
• Rebased to the last commit of the target branch (or merged it into my branch)
• Documented the changes
• Linked the issues which this PR resolves
• Checked the TODO section in README.md if this PR resolves it
• Updated the tests
• All tests are passing - cargo test