feat: Add Decimal Get/Set utilities

[paleolimbot]

Closes #170.

This is not designed to be a fully-featured performant decimal math library, but is designed to help with byte shuffling and provide enough support that users can write tests for decimal arrays (e.g., by providing integer getters and setters for numbers that fit within the int64 range).

Adds a struct ArrowDecimal and utility functions:

static inline void ArrowDecimalInit(struct ArrowDecimal* decimal, int32_t bitwidth,
                                    int32_t precision, int32_t scale);
static inline int64_t ArrowDecimalGetIntUnsafe(struct ArrowDecimal* decimal);
static inline void ArrowDecimalGetBytes(struct ArrowDecimal* decimal, uint8_t* out);
static inline int64_t ArrowDecimalSign(struct ArrowDecimal* decimal);
static inline void ArrowDecimalSetInt(struct ArrowDecimal* decimal, int64_t value);
static inline void ArrowDecimalSetBytes(struct ArrowDecimal* decimal,
                                        const uint8_t* value);

...and adds a "getter" and an "appender":

static inline ArrowErrorCode ArrowArrayAppendDecimal(struct ArrowArray* array,
                                                     struct ArrowDecimal* value);
static inline void ArrowArrayViewGetDecimalUnsafe(struct ArrowArrayView* array_view,
                                                  int64_t i, struct ArrowDecimal* out);

[codecov-commenter]

Codecov Report

Merging #180 (c6bb04a) into main (20e5bae) will decrease coverage by 0.01%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##             main     #180      +/-   ##
==========================================
- Coverage   93.43%   93.42%   -0.01%     
==========================================
  Files           7        7              
  Lines        1767     1765       -2     
  Branches       57       57              
==========================================
- Hits         1651     1649       -2     
  Misses         84       84              
  Partials       32       32              

Impacted Files	Coverage Δ
src/nanoarrow/array.c	93.29% <ø> (-0.03%)	⬇️
src/nanoarrow/array_inline.h	76.92% <ø> (ø)
src/nanoarrow/nanoarrow_types.h	92.30% <ø> (ø)

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[wjones127]

This looks good. Very clean implementation :)