Extend ColumnMapper to support join indices when reading from hipscat

[dougbrn]

Feature request
Improves #404. When reading from hipscat, the user must specify the join index for both object and source to join the sources to the object-level _hipscat index. This can easily be information that the ColumnMapper can hold and we can add this information to our known maps, allowing users who specify a map like "ZTF" to already have that join information available.

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