Make Telemetry API log record type generic

[raphael-theriault-swi]

📬 Issue #, if available: #977

✍️ Description of changes:

This adds a generic type parameter to LambdaTelemetryRecord for logs to support structured JSON logs. I initially tried to implement the RawValue newtype based solution suggested in #977, but it turns out you can't use RawValue within #[serde(flatten)].

This is a breaking change given it introduces a new generic parameter, but IMO it's better than introducing a whole new set of duplicate types, especially given supporting managed instances may already require breakage soon.

Unlike suggested in #977, this doesn't lock users into either supporting text or JSON logs at compile time, since they can just use an untagged enum:

#[derive(Deserialize)]
#[serde(untagged)]
enum LogRecord {
    Text(String),
    Json(StructuredLogRecord),
}

I also opted against adding a new struct for JSON logs since they don't have a fixed schema, some extension authors might want to use serde_json::Value, some might want a more specialized type.

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