Introduced a Catalog entry in the configuration struct. With it,
it's possible to control the maximum amount of entries returned
by /v2/catalog (`GetCatalog` in registry/handlers/catalog.go).
It's set to a default value of 1000.
`GetCatalog` returns 100 entries by default if no `n` is
provided. When provided it will be validated to be between `0`
and `MaxEntries` defined in Configuration. When `n` is outside
the aforementioned boundary, an error response is returned.
`GetCatalog` now handles `n=0` gracefully with an empty response
as well.
Signed-off-by: José D. Gómez R. <1josegomezr@gmail.com>
4c1561e9fb