Closes #2185 additional implementation of keep_source_vars

[zdz2101]

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[github-actions]

Package	Line Rate	Health
admiral	98%	✔
Summary	98% (4587 / 4673)	✔

[bms63]

I feel like the vignettes need to be updated that use this function

[bundfussr]

Using keep_source_vars here is different from the other functions. Usually it is used to specify the variables which should be kept from the selected records. To be consistent we would need to add keep_source_vars to event_source() and censor_source(). However, I am not sure if this is a good idea (especially if the default is exprs(everything()) because different source datasets could be used. For example if event_source() uses ADRS and censor_source() uses ADSL, all ADRS and all ADSL variables would be added to the new records but the ADSL variables would be populated for censored records only.

Therefore I think we should not add keep_source_vars to derive_param_tte().

@bms63 , what do you think?

[zdz2101]

@bms63 @bundfussr I think it's fine if we don't implement it here too, derive_param_tte() seems hyperspecific for users, in which case we can just close the PR as well

[bms63]

I am fine to not implement this change. @zdz2101 when you return - can you make a note of this in the function re-work documents.