feat: cli for getting tenants

[fabenan-f]

This PR adds an admin API endpoint for retrieving tenants and a corresponding CLI command. To provide better visibility/discoverability, the CLI uses a table format with text/tabwriter to display which tenants have been created (this can later be extended to show tenant status as well).

kr get tenants
ID                                    NAME      LABELS                     CREATED  UPDATED
aa528c4e-8308-4d3d-8beb-41194fec4814  cocasola  env=prod,team=powerran...  4m ago   4m ago
fc7c2b71-361c-4a8c-beed-cf7921889964  bluecow   env=dev,team=rocket        3m ago   3m ago

Out of scope for this PR:

• pagination and filtering of the tenant list
• better solution to display labels (right now they are truncated after 25 chars)