Allow SQL expression as value for sql_format

I think it could be useful for example to allow to write the CurrentTimestamp of the transaction (instead of the python datetime now).

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The module account_invoice_history could use it to store as the open date the transaction timestamp.

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assigned to @ced

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review279431002 updated at https://codereview.tryton.org/279431002/#ps265491002

review285241002 updated at https://codereview.tryton.org/285241002/#ps265491003

review279431002 updated at https://codereview.tryton.org/279431002/#ps271491003

New changeset 8c235c23550b by Cédric Krier in branch 'default':
Use SQL CurrentTimestamp to fill numbered_at
https://hg.tryton.org/modules/account_invoice_history/rev/8c235c23550b

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closed

New changeset e4f619ba9c25 by Cédric Krier in branch 'default':
Allow SQL expression as value of fields
https://hg.tryton.org/trytond/rev/e4f619ba9c25

New changeset c45236c291fb by Cédric Krier in branch 'default':
Allow SQL expression as value of fields
https://hg.tryton.org/tryton-env/rev/c45236c291fb