feat(shares): Introduce way to query # of completed samples for last block

[distractedm1nd]

          I just think there should be a way to query how many samples have been completed for the latest block as it could be useful for UXes or monitoring tools.

Currently it's possible to infer this with this endpoint (in a slightly different format, as probability), but if we remove that, sounds like there's no way of getting that.

Originally posted by @musalbas in #2769 (comment)

Right now, there is no way of doing this given our current call stack - we do not keep the information of how many samples have been completed for each block, including the last block. In addition, this amount is always constant and shown in the config - so if a block has been sampled (which can be seen via SharesAvailable), it means it has sampled SampleAmount shares.

So this endpoint should be introduced when we implement dynamic sampling #, which we should consider while designing ipldv2