[rust-reqwest]: group operations into structs

[maxnachlinger]

Fixes: #18637

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[maxnachlinger]

This change groups the operations against, in this case, the DefaultApi into a DefaultApi struct for organizational purposes.

[maxnachlinger]

This is a preference for sure :) For my part I find Rust code easier to maintain when it is grouped into meaningful structs. I would also be happy to add a feature to control this grouping behavior if we'd like to go that direction.