feat: String Validity Metric

[subhankarb]

Tell us about the problem you're trying to solve

New validity metric for String Columns

Describe the solution you’d like As a part of String validity metric we need to implement 3 different kind of metrics

• max_string_length
• min_string_length
• avg_string_length

The metric will generate the maximum/min/average value of the column's length.

The configuration example will be like below

metrics:
  - name: example_max_string_length
    metric_type: max_string_length
    resource: dcs_pgsql_db.dcs_iris.species
  - name: example_min_string_length
    metric_type: min_string_length
    resource: dcs_pgsql_db.dcs_iris.species
  - name: example_avg_string_length
    metric_type: avg_string_length
    resource: dcs_pgsql_db.dcs_iris.species

duplicated #175