Don't use TLS from function instances to brokers by default

[lhotari]

Motivation

• CI is currently broken after enabling Pulsar Function testing in all configuration with Test Pulsar Functions in CI #434
• When TLS is enabled Function instances cannot communicate to the broker since Function instances don't currently have the custom TLS CA cert available.
• When the function worker runs on the broker, the configuration PF_tlsAllowInsecureConnection: "true" has no impact and it's not possible to configure tlsAllowInsecureConnection=true only for the function instance. The function worker uses the broker configuration for the tlsAllowInsecureConnection setting and changing that would have a larger impact.

Modifications

• Add new configuration tls.function_instance.enabled that defaults to false. This controls whether TLS is used from function instances to the broker.

Verifying this change

• Make sure that the change passes the CI checks.

[tisonkun]

Thank you!