Allow ColumnMapper Known Map strings to be supplied directly to loader functions

[dougbrn]

Feature request
Known maps offer some nice convenience for users to just create a column mapper object from a string (e.g. ColumnMapper().use_known_map("ztf")). It would be even better to just allow users to pass the string as an input to functions like this: from_parquet(column_mapper="ztf") instead of needing to create a ColumnMapper object up front.

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• I have described the purpose of the suggested change, specifying what I need the enhancement to accomplish, i.e. what problem it solves.
• I have included any relevant links, screenshots, environment information, and data relevant to implementing the requested feature, as well as pseudocode for how I want to access the new functionality.
• If I have ideas for how the new feature could be implemented, I have provided explanations and/or pseudocode and/or task lists for the steps.

[dougbrn]

Add some kind of output that lets the user know what mappings were assigned