Switch from TableEntity to ITableEntity in the non-generic repository for flexibility

[kzu]

We currently use TableEntity for out TableEntityRepository. This limits the scenarios where you can use this repository, since it would not allow reading additional properties that may exist in the table, and also would not allow entity merging behavior like #46 introduced.

By switching the public API to ITableEntity, we would remain API-compatible with the previous version, but using DynamicTableEntity implementation internally would allow saving/merging additional properties as needed.