Support column type lengths than only STRING(MAX) and BYTES(MAX).

[bgood]

This should strip away the length of the column for variable length columns (STRING and BYTES). That would make it possible to use columns with a different length than only STRING(MAX) and BYTES(MAX).

(And yes, I know it could be easily circumvented by adding the column to the dynamodb_adapter_table_ddl as STRING(MAX), even if the column is for example STRING(100), but that will probably confuse users.)

Originally posted by @olavloite in #3 (comment)