DRILL-7117: Support creation of equi-depth histogram for selected data types.
Support int/bigint/float4/float8, time/timestamp/date and boolean.
Build the histogram from the t-digest byte array and serialize as JSON string.
More changes for serialization/deserialization.
Add code-gen stubs (empty) for VarChar/VarBinary types.
Address review comments (part 1). Add unit test.
Address review comments (part 2) for sampling.
close apache/drill#1715
