Allow multiple certificates in CACertificateFile

[bgrainger]

The Amazon Aurora documentation references a "bundled" certificate file: rds-combined-ca-bundle.pem.

This file is simply concatenated certificates:

-----BEGIN CERTIFICATE-----
MIID9DCCAtygAwIBAgIBQjANBgkqhkiG9w0BAQUFADCBijELMAkGA1UEBhMCVVMx
[ snip ]
/40NawZfTUU=
-----END CERTIFICATE-----
-----BEGIN CERTIFICATE-----
MIIEATCCAumgAwIBAgIBRDANBgkqhkiG9w0BAQUFADCBijELMAkGA1UEBhMCVVMx
[ snip ]
VPrXn2899T1rcTtFYFP16WXjGuc0
-----END CERTIFICATE-----
-----BEGIN CERTIFICATE-----
MIIEATCCAumgAwIBAgIBRTANBgkqhkiG9w0BAQUFADCBijELMAkGA1UEBhMCVVMx
...

It comprises a (self-signed) root certificate and multiple intermediate certificates. We need both the root and the intermediate certificates to verify the certificate chain for an Aurora SSL connection (see #498).

Since Amazon already makes this bundled certificate available, and creating a bundled certificate isn't hard, it makes most sense to allow the existing CACertificateFile connection string option to point to a bundled certificate file (as opposed to interpreting it as a delimited list of file names). Currently new X509Certificate2(string) only loads the first certificate in the file, so we would need to manually split the file into multiple byte[] and create multiple X509Certificate2 objects.