Add argument for token expiry date

The current `/token` endpoint does not provide a way for clients to specify the lifetime of the token to be generated. This should be added as a way to override the internal API default expiry value.

- [ ] expiry date / time added to the `/token` endpoint
- [ ] unit tests updated accordingly
- [ ] end-to-end tests updated accordingly
- [ ] `--expiry` option added to `kci user token` and underlying API Python bindings in `kernelci-core`

Start at the `/token` endpoint and inspect the existing unit and end-to-end tests. Then trace `kci user token` and its underlying API Python bindings in `kernelci-core`; done means the expiry override works across the endpoint, bindings, CLI option, unit tests, and end-to-end tests.

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