Add withdrawal support for notify_refund_pending RPC method

[Ifropc]

Description

Add support for withdrawals for existing notify_refund_pending RPC method

Context

I noticed refunds are not available for pending_user_transfer_complete and pending_external statuses.

Generally speaking, in the regular flow transaction goes from pending_user_transfer_start -> pending_anchor -> completed
However, when the anchor takes times to process a transaction and it's still in pending_anchor , user wants to refund it and can do so.
In the cases where status is pending_user_transfer_complete (funds are available for pickup) or pending_external (funds are processed by external entity) we want to fist move it to pending_anchor to indicate that funds are not available for pickup anymore, (or that bank transfer was cancelled), i.e. indicate that refund has been initiated.
In this case, if we have issues with Stellar transfer or Circle, etc. transaction won't be in status that shows user wrong information. (Especially in pending_user_transfer_complete case, where user funds is technically no longer available for pickup anymore, but status doesn't show that)
After that, we can use existing notify_refund_send RPC.

Testing

• ./gradlew test
• Added new test cases

Documentation

Flow charts should be updated (https://stellarorg.atlassian.net/browse/ANCHOR-688)

Known limitations

N/A

[github-actions]

Code Coverage

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[philipliu]

The code looks good although we should update the Platform API tests to cover the new flows.

[lijamie98]

LGTM with some minor comments.

[lijamie98]

Can we put a little details here so that we don't need to go back to the ticket?

[Ifropc]

Agree, let's do this little TODOs later on to not forget things that were put away!