Add support for Raw Public Key credentials

[aryan-25]

Motivation:

This PR decomposes the existing TLSCredentials type to separate out credential type (X509), source (file, byte array, or certificate reloader), and encoding (PEM or DER) from one another. This allows us to cleanly add support for a new TLS credential type, namely, Raw Public Keys (RFC 7250), which is currently only available over HTTP/3.

Modifications:

Updated TLSCredentials to now be structured around "credential type" as the top-level. TLSCredentials now provides these static factory methods (public):

• .x509(X509Credentials)
• .rawPublicKey(RawPublicKeyCredentials)

Internal API:

• X509Credentials.Backing:

  • .certificates
  • .reloading
  • .serialized(SerializedCredentials).

• SerializedCredentials:

  • a source (.file / .bytes) combined with a SerializationFormat (.pem / .der).

Internally, we can now represent TLS credentials in a more structured manner. These details are not exposed in the public API though. The public API just contains these convenience factory methods:

• X509Credentials:

  • .certificates(chain:privateKey:): in-memory X509.Certificate / Certificate.PrivateKey objects;
  • .reloading(_:): a CertificateReloader;
  • .pemFile / .pemBytes / .derFile / .derBytes: a serialized certificate chain and private key.

• RawPublicKeyCredentials:

  • .derFile.

Result:

Configuration API improved and support for Raw Public Key credentials over HTTP/3 added.