feat(sdk): add optional OTLP tracing via tracing config option (#241)

[Ademiitura]

Summary

Implements #241 — optional OTLP tracing for SDK consumers self-hosting an OTel collector.

Changes

• sdk/src/telemetry/tracing.ts (new) — no-op tracer by default, OTLP/HTTP flush when configured
• sdk/src/useInvisibleWallet.ts — added tracing option to WalletConfig, wraps register, deploy, and signAuthEntry with spans
• sdk/src/index.ts — exports TracingConfig type and shutdownTracing
• sdk/package.json — adds OTel packages as optional dependencies

Usage

const wallet = useInvisibleWallet({
  factoryAddress: FACTORY_CONTRACT_ID,
  rpcUrl: 'https://soroban-testnet.stellar.org',
  networkPassphrase: Networks.TESTNET,
  tracing: {
    exporter: 'otlp-http',
    endpoint: 'http://localhost:4318/v1/traces',
  },
});

Acceptance criteria

• Default off, no perf cost — no-op path has zero overhead when tracing is not configured
• When on, traces full payment lifecycle — spans cover veil.register, veil.deploy, veil.signAuthEntry
  Closes OTLP traces from SDK #241

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