ClientType: Change from &'static str to String

[kevinji]

Fixes #206.

Description

&'static str makes it impossible to deserialize ClientType and affects any structs including ClientType as well, so change it to use String instead.

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PR author checklist:

• Added changelog entry, using unclog.
• Added tests.
• Linked to GitHub issue.
• Updated code comments and documentation (e.g., docs/).
• Tagged one reviewer who will be the one responsible for shepherding this PR.

Reviewer checklist:

• Reviewed Files changed in the GitHub PR explorer.
• Manually tested (in case integration/unit/mock tests are absent).

[plafer]

Thank you for opening this PR. Could you:

1. Open an issue which clearly describes the problem
2. Mark this PR as closing this issue (i.e. add a line "Closes: #")
3. Refer to this issue in the changelog

Specifically, I am not sure what you mean by "making it impossible to deserialize". For example, this works:

let deserialized: ClientType = serde_json::from_str("\"hello\"").unwrap();
println!("Deserialized: {:?}", deserialized);

Please expand on that in the issue. Thanks!

[kevinji]

@plafer I added #206 to describe the issue further. I think your specific example works because "\"hello\"" happens to be a &'static str as it is a literal. If you instead had a function that took in a &str it would not work, as detailed in the issue.

[plafer]

Thank you for the additional context! LGTM.