Change `GET /rounds/meridian/{contract}/{round-index}` to be more restful

[bajtos]

See the discussion in https://www.notion.so/Moving-IPNI-queries-to-Spark-checkers-c58175c0f1c841e6bb82ac44a1472a98?d=66562e2c7ee94dedaf835725fb630abd&pvs=4#22453399ef9d4c3580be125837dcb8c5

would it be more restful to have this be meridian/contracts/{contractId}/rounds/{roundId} ?

Maybe even /meridian/{contract-address}/rounds/{round-id}

will meridian ever have other models exposed through its api? I’m just wondering if it is actually useful to have /contracts in the URL, in case in the future we have something like meridian/tasks/{taskId} or something
if I have a URL that says meridian/1234 im not sure what this value refers to

cc @patrickwoodhead