🐛[Bug] - When running in read-only mode, a write ledger is still required in the ledgers.yaml configuration

[esune]

When running ACA-Py in read-only mode (i.e.: we do this for verifiers such as VC-AuthN) with a multi-ledger configuration, the ledgers.yaml configuration file still requires a ledger to be set to is_write: true.

aca-py-1         |   File "/home/aries/.local/lib/python3.9/site-packages/aries_cloudagent/__main__.py", line 69, in script_main
aca-py-1         |     main(sys.argv)
aca-py-1         |   File "/home/aries/.local/lib/python3.9/site-packages/aries_cloudagent/__main__.py", line 75, in main
aca-py-1         |     run(args)
aca-py-1         |   File "/home/aries/.local/lib/python3.9/site-packages/aries_cloudagent/__main__.py", line 64, in run
aca-py-1         |     run_command(command, args)
aca-py-1         |   File "/home/aries/.local/lib/python3.9/site-packages/aries_cloudagent/commands/__init__.py", line 38, in run_command
aca-py-1         |     module.execute(argv)
aca-py-1         |   File "/home/aries/.local/lib/python3.9/site-packages/aries_cloudagent/commands/start.py", line 49, in execute
aca-py-1         |     settings = get_settings(args)
aca-py-1         |   File "/home/aries/.local/lib/python3.9/site-packages/aries_cloudagent/config/argparse.py", line 93, in get_settings
aca-py-1         |     raise e
aca-py-1         |   File "/home/aries/.local/lib/python3.9/site-packages/aries_cloudagent/config/argparse.py", line 90, in get_settings
aca-py-1         |     settings.update(group.get_settings(args))
aca-py-1         |   File "/home/aries/.local/lib/python3.9/site-packages/aries_cloudagent/config/argparse.py", line 944, in get_settings
aca-py-1         |     raise ArgsParseError(
aca-py-1         | aries_cloudagent.config.error.ArgsParseError: No write ledger genesis provided in multi-ledger config

This is misleading, and may/will lead to issues when configuring the service, as well as potentially when consuming the configuration file in the code - when running in read-only mode there should be no need to specify a is_write ledger.