Change AppUncheckedExtrinsic TryFrom to work with reference, since subxt can't return owned instance (and prevents clone()).

This is needed to avoid some situations in avail-test, where we only get reference. And it also works with owned case, so it's more leniant.

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[x] I have performed a self-review of my own code.
[x] The tests pass successfully with cargo test.
[x] The code was formatted with cargo fmt.
[x] The code compiles with no new warnings with cargo build --release and cargo build --release --features runtime-benchmarks.
[x] The code has no new warnings when using cargo clippy.
[x] If this change affects documented features or needs new documentation, I have created a PR with a documentation update.