make otel json formatter Value type generic over serde_json::Value

See: https://github.com/dylibso/observe-sdk/blob/main/rust/src/adapter/otel_formatter/mod.rs#L33C1-L38

If it makes sense to go more generic than `serde_json` then let's do it, but for this adapter we at least need to support json, so the `serde_json` option is fine

Start in rust/src/adapter/otel_formatter/mod.rs at lines 33–38 and inspect how the formatter currently uses serde_json::Value. Determine the smallest generic change that still supports JSON for this adapter; done means the formatter accepts the intended generic Value type while retaining serde_json compatibility.

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