…ey validation

Replaces node-forge dependency with Node.js native crypto module for
validating PEM-formatted private keys in ServiceAccount credential.

## Motivation

1. **License concerns**: node-forge contains code sourced from StackOverflow
   which is licensed under CC-BY-SA 4.0, causing issues with license scanners.
   See: digitalbazaar/forge#1001

2. **Security**: Native crypto is maintained by Node.js core team and uses
   OpenSSL's battle-tested C++ implementation.

3. **Performance**: Native crypto is significantly faster than the pure
   JavaScript implementation in node-forge.

4. **Bundle size**: Removes ~500KB dependency from the package.

## Changes

- Replace `forge.pki.privateKeyFromPem()` with `crypto.createPrivateKey()`
- Remove `node-forge` from dependencies
- Both functions throw on invalid PEM format, maintaining equivalent behavior