Conditional Client API token

## Problem
By default, Algorand public APIs require an algod_token to be consumed. However, it also allows the option to disable API authentication. When API auth is disabled, the request to the algod node can't be done since the algod token is always required by the client in go-algorand-sdk

## Solution
A potential solution could be to include a condition on [client.apiToken](https://github.com/algorand/go-algorand-sdk/blob/a55d63f27569304696d4e49971e3342e0b6bed21/client/v2/common/common.go#L157)
```
if len(client.apiToken) > 0 {
req.Header.Set(client.apiHeader, client.apiToken)
}
```