Upload with xet by default

[coyotte508]

Enable xet by default when available for uploads

Use the new git xet transfer to remove the API call to model/dataset/space info:

`${params.hubUrl ?? HUB_URL}/api/${repoId.type}s/${repoId.name}?expand[]=xetEnabled`,

cc @Wauplin @hanouticelina for viz ⬆️

We could also save more API calls by using the token returned by the git xet transfer - and remove a hardcoded value for the write token url. But I'm waiting until the git xet transfer protocol is stable before doing so - see also this comment - for now only using presence/absence of xet transfer in the response cc @seanses

cc @jsulz @assafvayner for viz

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For downloads, would like to do some perf tuning / analysis before enabling by default