Add compressMetadata and decompressMetadata to compression.ts

[Kingsman-99]

Description

src/compression.ts exists but has no standardised encode/decode for invoice metadata objects that need to be stored compactly (e.g. in a Stellar transaction memo or IPFS payload). This issue adds a JSON + base64url round-trip helper with size validation.

Acceptance Criteria

• compressMetadata(obj: Record<string, unknown>): string exported — serializes to JSON then encodes as base64url (no padding =)
• decompressMetadata(encoded: string): Record<string, unknown> exported — decodes base64url and parses JSON; throws SdkError with CONTRACT_REJECTED code on invalid input
• compressMetadata throws SdkError with CONTRACT_REJECTED code if the resulting base64 string exceeds maxBytes (default 512 bytes, configurable via second argument)
• Round-trip: decompressMetadata(compressMetadata(obj)) deep-equals obj for any JSON-serialisable input
• Both exported from src/index.ts
• Unit tests: round-trip correctness, oversized payload throws, invalid base64 input throws, empty object round-trips

Context

• Target files: src/compression.ts, src/index.ts
• Use Buffer.from(json).toString('base64url') — no external compression library

[ZacLou]

Hi @Emmanuel-Chukwunyere, I noticed PR #782 was closed without merging. Could you share what was missing or if you'd prefer a different approach? Happy to revise and resubmit.