"value" cannot be passed as a hex string on a SendTransaction request

If I have a smart contract function that is `payable` I need to set a `value` field in the JSON object of the request body for a `SendTransaction` request.

Today if I set this to a string like this:

```json
{
"value": "0x01"
}
```

I get the following result in the FireFly explorer:

```json
{
"id": "default:25a46a6d-961a-47e2-a7e0-5e3aaeb431f3",
"message": "json: invalid number literal, trying to unmarshal \"\\\"0x01\\\"\" into Number",
"success": false
}
```
I think some of the backslash escaping craziness there is actually just added by the UI.

Ethconnect logs the following:

```
[2022-06-15T12:57:33.507Z] WARN Failed to process message MsgContext[SendTransaction/default:25a46a6d-961a-47e2-a7e0-5e3aaeb431f3]: json: invalid number literal, trying to unmarshal "\"0x01\"" into Number
```