Calling WalletApi.walletBoxesCollect throws an exception

I am trying to use the new `WalletApi` to get some boxes from my wallet.

I created the wallet service like this
```scala
// conf is of type ErgoToolConfig
val apiClient = new ApiClient(conf.getNode.getNodeApi.getApiUrl, "ApiKeyAuth", conf.getNode.getNodeApi.getApiKey)
val walletService = apiClient.createService(classOf[WalletApi])
```

Then I'm using it like this to get some amount of ERG from my wallet
```scala
val getWalletBoxesRequest = new BoxesRequestHolder().targetBalance(Parameters.OneErg)
val response = walletService.walletBoxesCollect(getWalletBoxesRequest).execute()
```

But that call throws the following error:
> `Exception in thread "main" java.lang.IllegalArgumentException: Non-body HTTP method cannot contain @Body.`

After a little digging, I found that `WalletApi.walletBoxesCollect` queries `/wallet/boxes/collect` in the node's API, _which is a **POST** endpoint_.

However, I noticed that [the appkit method](https://github.com/ergoplatform/ergo-appkit/blob/develop/java-client-generated/src/main/java/org/ergoplatform/restapi/client/WalletApi.java#L152) that queries that endpoint has a `@GET` annotation for it

It _seemed_ like the fix was as simple as changing the annotation from `@GET` to `@POST`, so I tried it locally. For simplicity's sake, I created a Java interface in my codebase,
```java
public interface NewWalletApi {
@Headers({
"Content-Type:application/json"
})
@POST("wallet/boxes/collect")
Call> walletBoxesCollect(
@retrofit2.http.Body BoxesRequestHolder body
);
}
```

updated the wallet service creation to use the interface,
```scala
val walletService = apiClient.createService(classOf[NewWalletApi])
```

and tried executing a request again.
```scala
val getWalletBoxesRequest = new BoxesRequestHolder().targetBalance(Parameters.OneErg)
val response = walletService.walletBoxesCollect(getWalletBoxesRequest).execute()
```

This time, I just got a `Bad Request` back from the node, no further details.

So I went to the node's Swagger page to inspect that endpoint further (`POST /wallet/boxes/collect`), and soon learned some things about the expected request body.


1. `targetAssets` is required and cannot be `null`.
1. In the Swagger example, `targetAssets` is shown as a List of Lists (the `BoxesRequestHolder` Java model [reflects this](https://github.com/ergoplatform/ergo-appkit/blob/315c6d36955756e0f78da63e214a12d4221568b2/java-client-generated/src/main/java/org/ergoplatform/restapi/client/BoxesRequestHolder.java#L31)).
1. Sending a request, via the Swagger page, following this suggested structure _results in a **400 - Bad Request**_.

I tried out a few combinations
```json
// empty list
{
"targetAssets": []
...
}

// empty list with empty list
{
"targetAssets": [[]]
...
}

// list of lists with "dummy" values
{
"targetAssets": [["", 0]]
...
}
```

None worked. Until finally I tried this, and it worked!
```json
{
// empty object/map
"targetAssets": {}
...
}
```


So, it looks like
1. `targetAssets` cannot be `null` but it _can_ be empty.
1. Despite the Swagger example, `targetAssets` needs to be an object/map (where each key/value pair represent asset_id/amount), **NOT** a list of lists (array of arrays).

So, I tried updating the `targetAssets` property in the `BoxesRequestHolder` model to a `HashMap`, executed another request _from my code using appkit_, but then I got _this error_:

_Note: to facilitate and speed up local testing of this, I created a new model `NewBoxesRequestHolder`, updated `targetAssets` as needed, and used it to build a request body instead of `BoxesRequestHolder`_

Click to expand

```java
public class NewBoxesRequestHolder {
@SerializedName("targetAssets")
private java.util.HashMap targetAssets = new HashMap<>();

@SerializedName("targetBalance")
private Long targetBalance = null;

public NewBoxesRequestHolder targetAssets(java.util.HashMap targetAssets) {
this.targetAssets = targetAssets;
return this;
}

public NewBoxesRequestHolder targetBalance(Long targetBalance) {
this.targetBalance = targetBalance;
return this;
}

public java.util.HashMap getTargetAssets() {
return targetAssets;
}

public void setTargetAssets(java.util.HashMap targetAssets) {
this.targetAssets = targetAssets;
}

public Long getTargetBalance() {
return targetBalance;
}

public void setTargetBalance(Long targetBalance) {
this.targetBalance = targetBalance;
}
}
```

> `Exception in thread "main" java.lang.IllegalStateException: Expected BEGIN_ARRAY but was BEGIN_OBJECT at line 1 column 2 path $
at com.google.gson.stream.JsonReader.beginArray(JsonReader.java:350)
at com.google.gson.internal.bind.CollectionTypeAdapterFactory$Adapter.read(CollectionTypeAdapterFactory.java:80)
at com.google.gson.internal.bind.CollectionTypeAdapterFactory$Adapter.read(CollectionTypeAdapterFactory.java:61)
at retrofit2.converter.gson.GsonResponseBodyConverter.convert(GsonResponseBodyConverter.java:39)
at retrofit2.converter.gson.GsonResponseBodyConverter.convert(GsonResponseBodyConverter.java:27)
at retrofit2.OkHttpCall.parseResponse(OkHttpCall.java:225)
at retrofit2.OkHttpCall.execute(OkHttpCall.java:188)`

While I've figrued out how to send a valid request to `POST /wallet/boxes/collect` via the node's Swagger page, _I have not yet managed to get it to work via appkit_.

I'm still into it but this is as far as I've made it. If anyone has any useful insights into how to potentially resolve this issue, they'd be greatly appreciated!