Implement quantization for Decimal type when encode

[NyanFisher]

Hello!

Description of the problem solved by this PR

The msgspec library has many useful features, but the current version lacks the ability to correctly quantize Decimal values during encoding. In applications related to finance and precise calculations, it is critical to take into account maximum accuracy and return values rounded to a specified precision. Without implementing this functionality, a complete transition from pydantic to msgspec is not possible.

Changes implemented in this PR

1. An optional decimal_quantize parameter has been introduced. When specified, all decimal.Decimal values will be rounded to a scale corresponding to this value using the Decimal.quantize() method.
2. Added an optional decimal_rounding parameter to set the rounding mode for quantizing decimal values. Standard modes from the Python decimal module are supported, such as ‘ROUND_DOWN’, 'ROUND_HALF_UP', etc. If the parameter is not specified, the standard mode ‘ROUND_HALF_EVEN’ is applied.
   This improvement enhances the functionality of the library and ensures full compatibility when transitioning to msgspec in financial and other applications that require precise control over the representation of decimal numbers.

Examples

decimal_quantize

import decimal

encoder = msgspec.json.Encoder(decimal_quantize=decimal.Decimal("0.00"))

encoder.encode(decimal.Decimal("1.23456789"))
b'"1.23"'

decimal_rounding

import decimal

encoder = msgspec.json.Encoder(
    decimal_quantize=decimal.Decimal("0.00"),
    decimal_rounding=decimal.ROUND_UP,
)

encoder.encode(decimal.Decimal("1.235"))
b'"1.24"'

I would appreciate any comments on improving or restructuring the code, as I don't often write in C.

Fix my issue - #848