Update certificate loader to support diferent patterns for loading collections of certificates

[danroth27]

Update the certificate loader to support the following scenarios:

{
    "Authentication": {
        "Identity": {
            "SigningCertificates": { // Multiple inline
                "MySigningCert1": {
                    "Source": "File",
                    "Path": "cert1.pfx",
                    "Password": "*****"
                },
                "MySigningCert1": {
                    "Source": "File",
                    "Path": "cert2.pfx",
                    "Password": "*****"
                }
            }
        }
    }
}

{
    "Authentication": {
        "Identity": {
            "SigningCertificates": { // Single inline
                "Source": "File",
                "Path": "cert1.pfx",
                "Password": "*****"
            }
        }
    }
}

{
    "Certificates": {
        "MySigningCert1": {
            "Source": "File",
            "Path": "cert1.pfx",
            "Password": "*****"
        },
    },
    "Authentication": {
        "Identity": {
            "SigningCertificates": "MySigningCert1" // Single referenced
        }
    }
}

{
    "Certificates": {
        "MySigningCert1": {
            "Source": "File",
            "Path": "cert1.pfx",
            "Password": "*****"
        },
        "MySigningCert2": {
            "Source": "File",
            "Path": "cert2.pfx",
            "Password": "*****"
        }
    },
    "Authentication": {
        "Identity": {
            "SigningCertificates": "MySigningCert1 MySigningCert2" // Multiple referenced, space separated
        }
    }
}

[cesarblum]

preview1? cc @muratg