Add support for account creation through call endpoint

[awfm9]

Coinbase needs to be able to create accounts through the call endpoint, since there is no endpoint that specifically addresses this issue.

[Ullaakut]

@awfm9 According to the Flow documentation, creating an account can be done by submitting a transaction. Isn't the Submit endpoint enough then?

https://docs.onflow.org/flow-go-sdk/creating-accounts/

[Ullaakut]

The answer is that currently, the Construction API only creates value transfer transactions and cannot be used to create accounts, even though technically it would be possible to submit a manually written transaction to the Submit endpoint.

What we need to do is to make the Call endpoint able to build and submit the transaction to create accounts.

For now this is the only thing it should do, so it can remain fairly simple and does not need much genericity.

Pending questions:

• How do we handle fees for the account creation transaction? Need to do some research.
• How do we handle the time until account creation? The account is not created until the block in which the transaction was executed becomes finalized. We need Coinbase's answer on this.

[Ullaakut]

This task is no longer high priority. I am no longer working on it.