Command to list funding sources currently linked to a group

[thekaveman]

Building on the API from #28 and the existing groups command (i.e. take into account filtering).

Usage

$ littlepay groups [-f GROUP_TERMS] funding_sources

Acceptance criteria

• A new sub command for groups is defined called funding_sources (or similar)
• The new sub command lists the funding sources currently linked to one or more groups
• The new sub command builds on the groups filter, listing funding sources for group(s) that match the given filter if any
• The new sub command uses the get_concession_group_linked_funding_sources() API endpoint implementation to get its data
• The new sub command prints the resulting GroupFundingSourceResponse to stdout (don't worry about csv output)
• Tests are written to cover the new sub command

Background

We define CLI commands in the littlepay/commands subdirectory: https://github.com/cal-itp/littlepay/tree/main/littlepay/commands

Each command has its own module/file in that directory:

• configure
• groups
• products
• switch

The littlepay/main.py module/file sets up the CLI argument parsing and is the entrypoint for the CLI.

This is blocked by the fix for #42, which should be done first. #42 is done