[DISCUSS] Base64-encoded key/cert in the ClusterData

[Zixuan Liu]

Hi all,

In the ClusterData, we have two types of the key/certificate, one is PEM,
and one is JKS.

I would like to discuss the bae64-encoded key/certificate in PEM format.

The Pulsar can only load the key/certificate by the file path. When
configuring the key/certificate, we must copy the key/certificate files to
each cluster. Sometimes duplicating certificates is a chore, so I want to
add base64-encoded format support.

Based-64 encoded:
```
brokerClientKeyFilePath="data:application/x-pem-file;base64,BASE64-ENCODED-KEY-FILE-BODY"
brokerClientCertificateFilePath="data:application/x-pem-file;base64,BASE64-ENCODED-CERTIFICATE-FILE-BODY"
brokerClientTrustCertsFilePath="data:application/x-pem-file;base64,BASE64-ENCODED-TRUST-CERTS-FILE-BODY"
```

File path:
```
brokerClientKeyFilePath="/path/to/KEY-FILE.pem"
brokerClientCertificateFilePath="/path/to/CERTIFICATE-FILE.pem"
brokerClientTrustCertsFilePath="/path/to/TRUST-CERTS-FILE.pem"
```

Please let me know what you think.

Thanks,
Zixuan