Supported signature algorithms and key types are not documented - using unsupported key type leads to CLI panicking

For signing enclave images `nitro-cli` supports signatures using ECDSA as supported by COSE Sign1 (described in the [RFC8152](https://datatracker.ietf.org/doc/html/rfc8152#section-8.1)).

The implementation for that is in https://github.com/awslabs/aws-nitro-enclaves-cose

When using other key types (e.g. RSA) `nitro-cli` will panic:

```
$ sudo nitro-cli build-enclave --docker-uri --output-file test.eif --private-key rsa.pem --signing-certificate cert.pem

Start building the Enclave Image...
Using the locally available Docker image...
thread 'main' panicked at 'called `Result::unwrap()` on an `Err` value: UnsupportedError("Non-EC keys are not supported")', /builddir/build/BUILD/aws-nitro-enclaves-cli-1.3.1/crates-dependencies/aws-nitro-enclaves-image-format/src/utils/mod.rs:298:18
note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace
```

The error is actually created in https://github.com/awslabs/aws-nitro-enclaves-cose/blob/main/src/crypto/openssl_pkey.rs#L62 and not properly propagated up through https://github.com/aws/aws-nitro-enclaves-image-format/blob/main/src/utils/mod.rs#L297-L301 to fail gracefully and provide a proper error message.

Further, this constraint on signing key types is not documented appropriately and needs to be called out.