Enhancement: support PEM format for client certificates

[hex2a]

For the certificate authority the cacert (PEM) or truststore parameter can be used,
for client certificates there is only the keystore is available, which only accepts .jks or .p12 files.
Please implement support for PEM format for client certificates. (similar to the CA config parameter)

[wkjun]

a long time has pass,this issue can be solved?