Deserializing a ClientType requires an input with static lifetime

[kevinji]

Summary of Bug

ClientType contains a &'static str, which makes it impossible to Deserialize ClientType without leaking memory.

Version

0.21.1

Steps to Reproduce

Here's a simplified example of the issue, reproduced below:

use serde::Deserialize;
use serde_json;

#[derive(Deserialize)]
struct WithStaticStr(&'static str);

fn from_json(json_str: &str) -> WithStaticStr {
    serde_json::from_str(&json_str).unwrap()
}

There's no way to make this code compile besides giving json_str a &'static str lifetime.

Acceptance Criteria

Fix ClientType to contain a String instead of a &'static str.

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