Skip duplicate data during copy from one catalog to another

[pbeaucage]

Following up from comment on #735, this PR provides extremely rudimentary duplicate skipping during a tiled.client.sync.copy. I think it makes sense to put this behavior behind a flag skip_duplicates and leave it False by default. Duplicate handling is a little tricky for performance; I think the scope of .copy() is pretty clear that it doesn't update nodes in the destination catalog if the source has changed (though this, of course, might eventually be desirable behavior).

(edit per @danielballan's suggestion below: this flag is now a str called on_conflict with implemented options 'error' (default), 'warn', and 'skip')

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[danielballan]

Just one suggestion to lay track for future extensions. Could we make the parameter on_conflict and make the value a str or StrEnum with values skip and error (default)? In the future we might add warn, overwrite, etc. More discussion is needed, and this can be put off for a future PR, but if we choose an enum instead of a Boolean flag we leave space for extensibility.