[FW][IMP] account: added method for processing extra fields in refund wizard

[fw-bot]

If we add an extra field in the refund wizard, e.g.
there was no good method to inherit in order to put it on
the refund before the refund was posted in case of the
cancel or modify case.

That is why we put this code in a separate method, so
we can still modify the refund with the information from
the wizard before being posted. (as it is e.g. sent through
the api when posted)

Description of the issue/feature this PR addresses:

Current behavior before PR:

Desired behavior after PR is merged:

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Forward-Port-Of: #41478

[fw-bot]

Ping @jco-odoo, @qdp-odoo
Cherrypicking f01b840 of source #41478 failed

stderr:

error: could not apply f01b8403017... [REF] account: added method for processing extra fields in refund wizard
hint: after resolving the conflicts, mark the corrected paths
hint: with 'git add <paths>' or 'git rm <paths>'
hint: and commit the result with 'git commit'

Either perform the forward-port manually (and push to this branch, proceeding as usual) or close this PR (maybe?).

In the former case, you may want to edit this PR message as well.

[jco-odoo]

@nla-odoo Can you check if we can at least update this for 12.3. It will work differently for later versions anyways.

[qdp-odoo]

@jco-odoo can you do something with that PR? It bugs me

[jco-odoo]

@qdp-odoo @nla-odoo We will need some time to do it, so I will close it for the moment. You can always take the branch again and do the fixes and create another PR. There should not be too many things to do. But there are for 13.0 and that is the primary focus.