RISCV64 RVV Support

Notable Changes:

• Added RISCV64 RVV support for both Base64 encode and decode fast paths, with automatic fallback to generic logic when needed.
• Introduced adaptive vector-length processing to improve portability and throughput across different RVV-capable CPUs.
• Added RISCV64-specific asm tests and generic-vs-RVV benchmarks for correctness and performance validation.
• Added a new riscv64-qemu GitHub Actions workflow and updated README/README-CN CI badges and architecture lists.
• CI now covers RISCV64 QEMU testing end-to-end, and the new RVV path is verified passing.

v0.8.0

Notable Changes

• Loong64 LSX optimization support.

Since v0.8.0+, the minimum required version of Go has been changed to v1.25+. If you are unable to upgrade your Go version, please continue using the older version.

v0.7.0

Notable Changes:

• upgrade go to at least 1.23.0

By now Go 1.24.0 has been released, and Go 1.22 is no longer supported
per the Go Release Policy (https://go.dev/doc/devel/release#policy).

v0.6.2

Notable Changes:

• s390x: reduce one VN call
• add doc details to DecodeString and AppendDecode
• Upgrade dependency

v0.6.1

Notable Changes:

• arm64: decode minor optimization #21

v0.6.0

Notable Changes:

• Supports encoding length in [16,48) with ARM64 NEON instructions #20
• Supports decoding length in [24,64) with ARM64 NEON instructions #20

v0.5.0

Notable Changes:

• s390x optimization #19

v0.4.0

Notable Changes:

• ppc64x optimization #18

v0.3.2

Notable Changes:

• sync golang SDK.
• upgrade dependency.

v0.3.1

Update README to add performance stat.