Ensure decimal statistics are written correctly in Rust

[wjones127]

We are currently writing the underlying i128 value, but we should be writing the equivalent floating point value.

Here is an example from Spark:

delta-rs/rust/tests/data/delta-1.2.1-only-struct-stats/_delta_log/00000000000000000002.json

Line 2 in 860e272

{"add":{"path":"part-00000-28925d3a-bdf2-411e-bca9-b067444cbcb0-c000.snappy.parquet","partitionValues":{},"size":5489,"modificationTime":1666652374000,"dataChange":true,"stats":"{\"numRecords\":1,\"minValues\":{\"integer\":1,\"double\":1.234,\"decimal\":-5.67800,\"string\":\"string\",\"date\":\"2022-10-24\",\"timestamp\":\"2022-10-24T22:59:34.067Z\",\"struct\":{\"struct_element\":\"struct_value\"},\"nested_struct\":{\"struct_element\":{\"nested_struct_element\":\"nested_struct_value\"}}},\"maxValues\":{\"integer\":1,\"double\":1.234,\"decimal\":-5.67800,\"string\":\"string\",\"date\":\"2022-10-24\",\"timestamp\":\"2022-10-24T22:59:34.067Z\",\"struct\":{\"struct_element\":\"struct_value\"},\"nested_struct\":{\"struct_element\":{\"nested_struct_element\":\"nested_struct_value\"}}},\"nullCount\":{\"integer\":0,\"null\":1,\"boolean\":0,\"double\":0,\"decimal\":0,\"string\":0,\"binary\":0,\"date\":0,\"timestamp\":0,\"struct\":{\"struct_element\":0},\"map\":0,\"array\":0,\"nested_struct\":{\"struct_element\":{\"nested_struct_element\":0}},\"struct_of_array_of_map\":{\"struct_element\":0}}}","tags":{"INSERTION_TIME":"1666652374000000","OPTIMIZE_TARGET_SIZE":"268435456"}}}