feat: Add environment variable support for DID registration

- Bash implementation of DID creation-registration
- Move admin credentials to .env file for better security
- Add .env.example with default values
- Add dotenv support to TypeScript, Python and Bash implementations
- Update dependencies to include dotenv packages
- Fix JWT signature format in Bash implementation
- Add DER signature parsing for ES256K in Bash script

The admin DID and private key can now be configured via environment
variables, with fallback to default values if not set. This allows
for easier testing and deployment across different environments.

.env.example

@@ -0,0 +1,6 @@
+# Admin DID credentials
+ADMIN_DID=did:ethr:0x0000694B58C2cC69658993A90D3840C560f2F51F
+ADMIN_PRIVATE_KEY=2b6472c026ec2aa2c4235c994a63868fc9212d18b58f6cbfe861b52e71330f5b
+
+# API Configuration
+ENDORSER_API_URL=https://test-api.endorser.ch/api/v2/claim