Core: Align snapshot summary property names for delete files

[aokolnychyi]

This PR renames recently added and unreleased snapshot summary properties for equality and position delete files to match other recent public constants/methods.

Specifically, all other snapshot properties use full words for equality and position.

// SnapshotSummary
public static final String ADDED_POS_DELETES_PROP = "added-position-deletes";
public static final String REMOVED_POS_DELETES_PROP = "removed-position-deletes";
public static final String TOTAL_POS_DELETES_PROP = "total-position-deletes";
public static final String ADDED_EQ_DELETES_PROP = "added-equality-deletes";

// ConvertEqualityDeleteFiles
int convertedEqualityDeleteFilesCount();
int addedPositionDeleteFilesCount();

// FileWriterFactory
EqualityDeleteWriter<T> newEqualityDeleteWriter(...);
PositionDeleteWriter<T> newPositionDeleteWriter(...);

etc

Since the new snapshot summary properties haven't been released yet and were added recently, I'd consider renaming them for consistency.

[aokolnychyi]

@chenjunjiedada @rdblue, what do you think?

[rdblue]

Looks good to me.

[chenjunjiedada]

+1 to me.

[aokolnychyi]

Thanks, @rdblue @szehon-ho @chenjunjiedada!