[rust client] upgrade reqwest crate to 0.11

[typester]

This p-r upgrades the reqwest crate to 0.11.

reqwest 0.11 uses tokio 1.0 and it should be totally preferable for most of tokio users.
Also, since 0.11 make multipart support optional, I added it to the features list.

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[typester]

@frol @farcaller @richardwhiuk @paladinzh

[wing328]

Thanks for the PR. I don't think it's a breaking change so I'll re-target the PR to master instead.

[wing328]

Looks like your PR is based on 5.1.x so I'll keep it that way to avoid merging those sample update (5.1.x) changes back in current master.

I'll merge this and cherry-pick the change into master instead.

[typester]

@wing328 No, I would say that this p-r includes a breaking change.

reqwest 0.11 depends on tokio 1.0, while reqwest 0.10 and earlier depends on tokio 0.2.
And tokio 1.0 and 0.2 cannot be used at the same time unless you use a compatibility layer such as tokio_compat.

If the current user is in the tokio 0.2 codebase, it would be less confusing to continue to use reqwest 0.10 for this library as well.

That's why I did this p-r for 5.1.x.

[wing328]

@typester thanks for the explanation. Let's keep this targeting 5.1.x

[wing328]

Travis tests passed via https://travis-ci.org/github/OpenAPITools/openapi-generator/builds/758662988

[wing328]

@typester thanks for the PR, which has been included in the v5.1.0 release: https://twitter.com/oas_generator/status/1373636654024380423