api/users: Allow string UIDs for the ExternalProvider API data.

The GitLab API expects and allows arbitrary strings as UID as some
providers have non integer UIDs for mapping. Reflect the API and struct
name according to the actual API.
18 jobs for !295 with fix/api-users-external-uid-string in 33 seconds (queued for 2 seconds)
detached
Status Name Job ID Coverage
  Prepare
canceled prepare:cargo-cache-newest #5815686
build docker linux

00:00:33

 
  Build
canceled build:cargo-clippy #5815687
build docker linux

canceled build:cargo-clippy-no-default-features #5815688
build docker linux

canceled build:cargo-minimum #5815689
build docker linux

canceled build:cargo-minimum-no-default-features #5815690
build docker linux

canceled build:cargo-nightly #5815694
build docker linux

canceled build:cargo-nightly-no-default-features #5815695
build docker linux

canceled build:cargo-stable #5815691
build docker linux

canceled build:cargo-stable-no-default-features #5815692
build docker linux

canceled build:cargo-tarpaulin #5815693
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  Test
canceled test:cargo-minimum #5815696
build docker linux

canceled test:cargo-minimum-no-default-features #5815697
build docker linux

canceled test:cargo-nightly #5815701
build docker linux

canceled test:cargo-nightly-no-default-features #5815702
build docker linux

canceled test:cargo-stable #5815698
build docker linux

canceled test:cargo-stable-no-default-features #5815699
build docker linux

canceled test:cargo-tarpaulin #5815700
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  External
passed ghostflow-check-master