Support free base duration for preapprovals

[moritzkiefer-da]

Heavily based/stolen from #1954

[ci]

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Cluster Testing

• If a cluster test is required, comment /cluster_test on this PR to request it, and ping someone with access to the DA-internal system to approve it.
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[moritzkiefer-da]

@meiersi-da I've rebased your PR and addressed some fixmes for more test coverage. Otherwise it still looks good to me and seems to solve the issue nicely without causing backwards compatibility headaches. Can I ask for a look on the Daml code before I adjust backends and start prepping a CIP?

[meiersi-da]

Looks good to me. Thanks for picking this up; and glad we had this just laying around.

[meiersi-da]

Thanks. Let me know when you need an approval.

[moritzkiefer-da]

should be ready now, obviously not gonna merge before the CIP gets accepted