fix(manifest): encode decimal partitions with declared precision

[officialasishkumar]

Summary

• track decimal partition precision separately from decimal scale while building manifest field metadata
• use the declared Avro decimal precision to determine fixed-size encoding width
• add regression coverage for a decimal(10, 2) partition value whose textual length is smaller than the required fixed size

Fixes #1027

Testing

• go test . -run TestManifest/TestWriteManifestDecimalPartitionUsesDeclaredPrecision -count=1
• go test . ./codec -count=1

[zeroshade]

can we combine the decimalScales and decimalPrecisions into a single that maps the fieldID to a combined scale and precision? Compacting that would be better than having two separate maps.