Add table split points into local configuration folder structure

### Background

Split from:
- https://github.com/gchq/sleeper/issues/3693

This was missed out during:
- https://github.com/gchq/sleeper/issues/267

### Description

Starting split points for a table are configured with a property that references a local file at an absolute path (`sleeper.table.splits.file`). We'd like to replace this with a file that sits in the table configuration folder.

### Analysis

This also needs to be updated in the deployment documentation.

Ideally we should also remove the property `sleeper.table.splits.base64.encoded`, as its purpose is just for applying `sleeper.table.splits.file`.