Support aggregate validation on string columns with length(string_col)

[nehanene15]

DVT should support column level aggregates (sum, min, max, avg) on the length of string columns. This is supported via calculated fields in a YAML config, but it should also be supported through the CLI.

Running a '*' aggregation should run over all numeric columns.
data-validation validate column -sc my_bq_conn -tc my_bq_conn -tbls pso-kokoro-resources.hivetest.mascot -sum '*'

But specifying a certain string column name should implicitly convert run the aggregation over the length(string_col)
data-validation validate column -sc my_bq_conn -tc my_bq_conn -tbls pso-kokoro-resources.hivetest.mascot -sum name

[ryanmcdowell]

Perhaps adding an additional flag could also help:

--wildcard-include-string-len

Thinking that it may be simpler to run a single validation run for cases which you want to do both numeric and string length instead of separate runs.