[c++/python] Handle DataFrame index types beyond int/string

[johnkerl]

Split out from #922

Currently:

• libtiledbsoma.cc only allows int and string indices: https://github.com/single-cell-data/TileDB-SOMA/blob/0.5.0a11/libtiledbsoma/src/pyapi/libtiledbsoma.cc#L328-L350
• The underlyling soma_reader.h / soma_reader.cc support templated types: https://github.com/single-cell-data/TileDB-SOMA/blob/0.5.0a11/libtiledbsoma/include/tiledbsoma/soma_reader.h#L152-L153

Therefore this should be a straightforward elaboration in libtiledbsoma.cc.

Work here will, along the way, address the unit-test checklist from #956.

Status:

• int64 and string -- status quo before this issue
• int32, int16, int8, uint64, uint32, uint16, uint8, float32, float64
  • Python [python] Handle DataFrame index types beyond int/string -- except timestamps and bools #968
  • R [r] Handle SOMADataFrame index types beyond int/string #984 -- tracked in a separate issue
• pyarrow.timestamp("s"), pyarrow.timestamp("ms"), pyarrow.timestamp("us"), pyarrow.timestamp("ns")
  • Python [python] Handle timestamp index types for DataFrame #985
  • R [r] Handle SOMADataFrame index types beyond int/string #984 -- tracked in a separate issue
• bool: TileDB currently does not support bool as an index type
  • This is a low-pri feature request for TileDB Embedded: SC 25805

[johnkerl]

Closing as all remaining issues have their own tracking tasks