Support multiple PEM encoded certificates

At the moment, PEM SslBundles can be instantiated through the following properties:

spring:
  ssl:
    bundle:
      pem:
        client:
          truststore:
            certificate: "classpath:client.crt"

Where client.crt can contain multiple certificates.

In some situations, multiple very different certificates need to be trusted. For instance:

• An external service changes the CA it uses to sign the certificate it exposes. Both the old one and the new one need to be trusted on the client side to avoid downtime.
• When exposing a service with mutual authentication enabled, and when the corporate CA is re-used for multiple systems, it can be somewhat easy to obtain a keypair for a given FQDN. Trusting the CA is not an option, the trust store must contain all the allowed certificates.

While concatenating all the trusted certificates in the same file is an option, it makes it quite hard to see at a glance which certificates are trusted, as they are PEM-encoded.
It would be nice to be able to use file names to identify the certificates:

spring:
  ssl:
    bundle:
      pem:
        client:
          truststore:
            certificates:
            - "classpath:allowed-client1.crt"
            - "classpath:allowed-client2.crt"