New Feature Request: Support Custom Managed Channel Factory in Spring Zeebe

[robcossin]

We have a requirement to customize our ZeebeClient managed channel more than what is supported by the Zeebe configuration properties. We have created a fork and would like to implement the below change to support this requirement and add this flexibility for other users of this component.

We'd like to:

• add a new bean type: ZeebeClientManagedChannelFactory that will enable customization of the ManagedChannel that is passed into ZeebeClient within the ZeebeClientProdAutoConfiguration.zeebeClient bean.
• This follows the same pattern as ZeebeClientExecutorService; the ZeebeClientManagedChannelFactory will have a default implementation (which uses the current logic) within ZeebeClientAllAutoConfiguration that will be created when there is no other bean of the same type defined.