Make TLS TrustStore Configurable for WebSocket Proxy and Admin HTTP Requests

[michaeljmarshall]

When running with burnell enabled as part of the Pulsar Proxy, I am seeing the following error:

2022/04/26 05:49:39 error GET brokers https://pulsar-broker.default.svc.cluster.local:8443/admin/v2/brokers/pulsar error Get "https://pulsar-broker.default.svc.cluster.local:8443/admin/v2/brokers/pulsar": x509: certificate signed by unknown authority app=broker stats cache

Based on a quick look through the source code, there is no support for configuring the root CA when using self signed certs. This is already support using the TrustStore config for the pulsar+ssl protocol.

The same appears to be true for the websocket proxying done in burnell, although I am not sure if this feature is needed any more.