Add comparison keys to plan string for in-union

[alecgrieser]

The comparisonKeyFunction is currently not included in the plan representation of InUnion plans. This can make it hard to inspect bugs that could be a result of an in-union improperly merging results from different legs of a union. This does make the plan representation a little more verbose, though hopefully manageably so. It should be less intrusive, for example, than a regular union, which puts the comparison key between each leg of the union (unless the comparison is by primary key).

[foundationdb-ci]

Result of fdb-record-layer-pr on Linux CentOS 7

• Commit ID: 6c5b982
• Duration 0:51:30
• Result: ✅ SUCCEEDED
• Error: N/A
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