[16.0] account_loan: Interest related account move lines and taxes

[enricostano]

Is your feature request related to a problem?
In some cases the interest rate defined at loan level is considered as gross, meaning that it doesn't represent the net quantity that the lender will receive. A classical scenario is that the company that pays the interest applies a tax retention mechanism.

If we configure a default tax for the interest account, for example like the following:

And we configure the loan like the following:

If we now look at the account move generated by the module for the first interest payment:

1. The 211000 move line amount should be the difference between the gross amount (75$) and the tax retention amount (14.25$) = 60.75$
2. The 101401 move line is not necessary

Describe the solution you'd like
In this cases, the account move generated by the loan module should correctly take in account the tax retention.

Describe alternatives you've considered
A clear and concise description of any alternative solutions or features you've considered.

Additional context
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