Make Subscription Source Type Optional

[asonnenschein]

This MR updates the Subscriptions SDK and CLI to make the subscription source type parameter optional. For catalog source subscriptions this is a simple change because the catalog source type was programmatically appended to the catalog source instead of requiring user input as a positional arg to catalog_source(). To make the catalog type optional then I simply removed the line in catalog_source() that appends catalog source type.

For planetary variable source subscriptions this change is a bit more difficult to make without introducing a breaking change, because the source type is variable and is the first required positional arg to planetary_variable_source(). To avoid the breaking change, I added a condition to planetary_variable_source() that checks if the var_type arg is None and only appends the source type if it is not None. This gives us the desired UX for the CLI which no longer requires the --var-type parameter, but keeps the positional var_type arg in planetary_variable_source().

Note - I did explore the possibility of making the positional var_type arg optional by counting the number of input args to planetary_variable_source(), checking their types, and inferring whether or not the user specified the var_type arg. While this does technically work, I think it would introduce an anti-pattern to the method because var_type is the first positional arg in the method.