`Txid` type is currently kind of a footgun

After trying to use the `Txid` type in the testing of [this pr]( https://github.com/bitcoindevkit/bdk-ffi/pull/598), I realized a few things I had not yet internalized.

1. The `Txid` type is just a typealias on `String` until you use it in a method that requires an actual `Txid`. At that point it attempts the conversion, and panics if it can't do it.
2. This panic is not catchable by the target languages.

## Example
If you use a valid `Txid`, the new type works perfectly, and allows type-safe API on a method like `get_tx(txid: Txid)`. Does the conversion, provides it to the Esplora client in this case, and everything is dandy.

```rust
pub fn get_tx(&self, txid: Txid) -> Result>, EsploraError> {
let tx_opt = self.0.get_tx(&txid)?;
Ok(tx_opt.map(|inner| Arc::new(Transaction::from(inner))))
}
```

```kotlin
// This works
val txid: Txid = "9d0a83df4a556d61338ff18cbc20bf49a3c732805e404a66e25e1579271597a9"
val transaction: Transaction? = esploraClient.getTx(txid)
println("Transaction: ${transaction?.computeTxid()}")
```

But if your `Txid` is not valid, you'll get a panic which is not thrown on the JVM side, and therefore cannot be caught.

```kotlin
This crashes your test/app
try {
val txid: Txid = "imatxidiswear"
val transaction: Transaction? = esploraClient.getTx(txid)
println("Transaction: ${transaction?.computeTxid()}")
} catch (t: Throwable) {
println("Panic: $t") // you'll never get here
}
```

I just realized this a few hours ago and haven't looked at how/whether we could solve it using the
```txt
[Custom]
typedef string Txid;
```

construction with a `Result` or if the only way is to have a proper constructor on the Txid type. I'll take a look at this next week!

Start by tracing the Txid typedef and its conversion at methods such as get_tx, then compare the proposed Custom typedef with a Result against a proper Txid constructor. Done means invalid Txid values produce an error that target languages can catch instead of an uncaught panic.

Written by the indexing model from the issue text.