[FEA] Generate unique keys table in java JNI contiguousSplitGroups

[res-life]

Is your feature request related to a problem? Please describe.
Table.contiguousSplitGroups splits a table into sub-groups, but each group's group-by key is not collected and de-duplicated.

contiguousSplitGroups already generated split indices, so it's efficient to collect and de-duplicate the group-by keys by invoking a gather.

Describe the solution you'd like
Generate an extra table to collect the unique keys corresponding to sub-groups.
Origin implementation example:

     * Example:
     *   Grouping column index: 0
     *   Input: A table of 3 rows (two groups)
     *             a    1
     *             b    2
     *             b    3
     *
     * Result:
     *   Two tables, one group one table.
     *   Result[0]:
     *              a    1
     *
     *   Result[1]:
     *              b    2
     *              b    3

New requirement example:

contiguousSplitGroups
     * Example:
     *   Grouping column index: 0
     *   Input: A table of 3 rows (two groups)
     *             a    1
     *             b    2
     *             b    3
     *
     * Result:  GroupByResult
     *   groups:  Two tables, one group one table.
     *          group[0]:
     *              a    1
     * 
     *          group[1]:
     *              b    2
     *              b    3
     *    uniqKeysTable: Two rows, one row is corresponding to one group.
     *      a  // for group 0
     *      b  // for group 1

Describe alternatives you've considered
A clear and concise description of any alternative solutions or features you've considered.

Additional context

In the NVIDIA/spark-rapids#5999, need to split the input into groups and then get the unique keys table to generate partition strings. We can generate the distinct group-by keys by t.groupBy(columnIds).aggregate() after contiguousSplitGroups, but this is inefficient. Here we can gather the split indices produced by contiguousSplitGroups to generate the unique keys.