Provide working request signing examples against `/test-signature` endpoint

[alexheretic]

It would be helpful to have runnable examples in each lang POST https://api.truelayer-sandbox.com/test-signature.

Example readme template:

$LANG request signature example

Sends a signed request to https://api.truelayer-sandbox.com/test-signature.

Run

Set environment variables:

• ACCESS_TOKEN A valid JWT access token for payments scope docs.
• KID The certificate/key UUID for associated with your public key uploaded to console.truelayer.com.
• PRIVATE_KEY Private key PEM string that matches the KID & uploaded public key.
  Should have the same format as this example private key.

$ $command-to-run-example
204 ✓

[alexheretic]

Java taken care of in #56