[Feature]: another example in solace-samples-go/howtos/secure_connection.go: client certificate authentication

[solace-brendabutler]

Feature Description

Please add an example for mTLS (client certificate authentication) in secure_connection.go

Internal references:
support 119638
SolaceProducts/pubsubplus-go-client#21

Use Case

• so customer can see how to do it
• so customer can test that the broker is set up correctly for mTLS with a known-working example.

Proposed Solution

Include sample client certificate chain, so customer can configure client-certificate-authority for the supplied client-certificate in the example.