⚠️ fail requests with invalid custom partitioning

[de-sh]

We need to require custom partitioning to be strict, disallowing keys/values that contain special characters.

          This requires that the custom partitioning should adhere to not have name/values that aren't alphanumeric. This point has to be documented well.

Originally posted by @de-sh in https://github.com/parseablehq/parseable/pull/1193#discussion_r1966026909