[PAYG-723] Adds rust tl-signature example

[tl-flavio-barinas]

#51 for Rust

Rust 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.

$ cargo run
204 No Content ✓

[alexheretic]

Lets add a link in the README too, like https://github.com/TrueLayer/truelayer-signing/blob/main/nodejs/README.md?plain=1#L25

[alexheretic]

Great! It would be good to have similar comments to what we have in the nodejs & C# ones too.

E.g. explaining each bit of the signing call and reiterating

// Request body & any signed headers *must* exactly match what was used to generate the signature.

Probably just worth copying those comments into the rust example in the right places.

[alexheretic]

great stuff 🎉

[alexheretic]

I'm not sure why only the java actions are running though, this happened on the recent C# pr too..but not the nodejs one 🤔