Allow clients to support reading decimals with wider bit-width

Today, decimal bit-width is entirely determined by Kudu. When creating a schema of a given precision and scale, Kudu determines the correct bit-width for the parameters, and uses that to store values.

Client scanners can only specify reading DECIMAL (ignorant of bit-width). In requesting the columnar layout, however, it'd be nice if client scanners could also specify the desired bit-width to get back from tservers, and have the tservers inflate values as appropriate. This would be helpful, e.g. to read DECIMAL32- and DECIMAL64-stored data in Arrow, which currently only supports DECIMAL128 and DECIMAL256.