feat(cbor): Add BigInteger support with RFC 8949 §3.4.3 compliance

[e-thoman]

Add CBOR encoding and decoding support for BigInteger using CBOR tags 2 (positive bignum) and 3 (negative bignum) as specified by RFC 8949 §3.4.3 and the Smithy RPC v2 CBOR protocol.

• Preferred serialization: values fitting in u64/i64 use major types 0/1 instead of bignum tags
• Tag 3 correctly encodes -1-n per RFC 8949
• Handle minicbor Int type for major type 1 values exceeding i64 range
• Add num-bigint dependency to aws-smithy-cbor
• Codegen: BigIntegerShape delegates to decoder.big_integer() / encoder.big_integer()
• BigDecimal remains unsupported with CBOR (fails at codegen time)
• Comprehensive tests: round-trip, RFC 8949 Appendix A interop, edge cases

Resolves: smithy-rs#4473

Motivation and Context

BigInteger was previously unsupported with the CBOR protocol — attempting to use a BigIntegerShape in a Smithy model with RPC v2 CBOR would fail at codegen time with a CodegenException.
This blocked any service or client that needed arbitrary-precision integers over CBOR. The Smithy RPC v2 CBOR spec requires BigInteger support via CBOR tags 2 and 3 (RFC 8949 §3.4.3), and this
change implements that requirement. See smithy-rs#4473.

Description

Rust runtime (aws-smithy-cbor):

• Added big_integer() to Encoder — writes values using preferred serialization (major type 0/1 for values fitting u64/i64, tags 2/3 for larger values). Tag 3 encodes the byte string as n
  where the value is -1 - n per RFC 8949.
• Added big_integer() to Decoder — reads CBOR tags 2/3 (bignum), plain unsigned/signed integers (major types 0/1), and minicbor Int type for major type 1 values exceeding i64 range.
• Added num-bigint 0.4 dependency for arbitrary-precision integer arithmetic.
• Added strip_leading_zeroes() helper for preferred bignum serialization.

Codegen (codegen-core):

• CborParserGenerator.kt: BigIntegerShape now emits decoder.big_integer() instead of throwing CodegenException.
• CborSerializerGenerator.kt: BigIntegerShape now emits encoder.big_integer() instead of throwing CodegenException.
• BigDecimalShape remains unsupported (still throws CodegenException).

Testing

• 18 new Rust unit tests in aws-smithy-cbor covering:
  • Round-trip encode/decode for positive, negative, and large values
  • RFC 8949 Appendix A interoperability vectors (2^64, -2^64-1)
  • Preferred serialization verification (small values use major types 0/1, not tags)
  • Edge cases: empty byte strings for tags 2/3, invalid tag rejection, major type 1 values exceeding i64
  • Leading zero stripping in bignum byte strings
• Updated Kotlin codegen tests to verify successful code generation instead of expecting CodegenException
• All 29 aws-smithy-cbor tests pass

Checklist

• For changes to the smithy-rs codegen or runtime crates, I have created a changelog entry Markdown file in the .changelog directory, specifying "client," "server," or both in the
  applies_to key.

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[lauzadis]

nit: this comment was accidentally detached from pub fn blob below

[lauzadis]

should we also test round-tripping of positive signed values here? (e.g. +123)