Write spark StringType columns as Utf8 YTsaurus type

[alextokarew]

It is needed to support writing Spark dataframes with StringType columns to YTsaurus tables with Utf8 type columns. Two ways should be supported:

1. df.write.option("string_to_utf8", "true").yt("//path/to/table") to convert all string columns to Utf8 type on writing;
2. import yt.type_info as ti; df.write.schema_hint({"some_string_column": ti.Utf8}) to convert explicitly specified fields to utf8

[alextokarew]

@faucct I think we've implemented it. Please link appropriate commit to this Issue and close it.

[faucct]

#26