Input formats: save unknown fields to a special column instead of skipping

[ksverdlov]

Use case
If input_format_skip_unknown_fields = 1 (skipping is enabled), ClickHouse doesn’t insert extra data. I need to save extra data in origin format to a special column (String-type) for further processing.

Describe the solution you'd like
Some additional option for format, in which I can pass a column name. In that column Clickhouse will save unknown/skipped fields in origin format.

For example,

INSERT INTO UserActivity FORMAT JSONEachRow(`skipped`) {"PageViews":5, "UserID":"4324182021466249494", "Duration":146,"Sign":-1,"SomeUnknownField":"StringValue"} {"UserID":"4324182021466249494","PageViews":6,"Duration":185,"Sign":1,"SomeOtherUnknownField":123}

Describe alternatives you've considered
Insert all JSON object to one String column with short TTL for latter processing. Unfortunately Clickhouse doesn`t have any functions to modify JSONs (to cut some fields, for example).