create update_sla() function to allow changing settings of an existing SLA

[KowMangler]

Is your feature request related to a problem? Please describe.

A clear and concise description of what the problem is. Ex. I'm always frustrated when [...]

When retention is required to be changed or any other setting (archive target, replication target) etc I have to resort to custom code and direct API calls.

Describe the solution you'd like

A clear and concise description of what you want to happen.

Create an update_sla function so I can use it to modify existing SLAs

Describe alternatives you've considered

A clear and concise description of any alternative solutions or features you've considered.

When retention is required to be changed or any other setting (archive target, replication target) etc I have to resort to custom code and direct API calls.

Additional context

Add any other context or screenshots about the feature request here.