Restore backward-compatible requestOptions for axios migration

[Copilot]

PR #278 replaced postman-request with axios but silently dropped support for TLS options (ca, cert, key, passphrase, agentOptions, strictSSL) passed via requestOptions. The README also documented httpsAgent/httpAgent/timeout forwarding that never actually worked.

Changes

• src/index.js: Map postman-request-style TLS options to https.Agent in the axios request wrapper. Config-level TLS options are baked into the default agent at init time; per-request agents are only created when per-call options differ (reference comparison). Also forward timeout, httpsAgent, httpAgent to per-request axios options.
• index.d.ts: Add TlsOptions interface (ca, cert, key, passphrase, pfx, strictSSL, agentOptions, timeout) merged with AxiosRequestConfig for requestOptions type.
• README.md: Replace stale postman-request reference with accurate description of supported options.
• example/auth.js: Remove followAllRedirects (silently ignored since axios migration).
• test/unit.js: 10 new tests covering TLS forwarding, agent caching, and option passthrough.

Usage

// This works again (was silently broken by #278)
const vault = require('node-vault')({
  requestOptions: {
    ca: fs.readFileSync('/path/to/ca.pem'),
  },
});

// Per-call overrides still work
vault.read('secret/data', { ca: otherCaCert });

// Axios-native options also forwarded
const vault2 = require('node-vault')({
  requestOptions: {
    httpsAgent: new SocksProxyAgent('socks://127.0.0.1:1080'),
    timeout: 5000,
  },
});

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