Add pattern type to allow datetime with timedelta

[chebertpinard]

At the moment the patterns supported in earthkit.data.utils.pattern allow to substitute a datetime to a specified format in a string. It would be useful to be able to apply a timedelta to the given datetime before the substitution.

Example:

In this case we have analysis data for a date:

/home/ecmv9074/hpcperm/cmc_data/2025011200_000
/home/ecmv9074/hpcperm/cmc_data/2025011206_000
...

path: /home/ecmv9074/hpcperm/cmc_data/{date:strftime(%Y%m%d)}{date:strftime(%H)}_000

But for this same validity date the file name pattern is 6h before for the forecast file. In that case the strftime can't be use:

/home/ecmv9074/hpcperm/cmc_data/2025011118_006
/home/ecmv9074/hpcperm/cmc_data/2025011200_006
...


path: /home/ecmv9074/hpcperm/cmc_data/{date:???}{date:???}_006

The idea would be to add a new pattern TYPE that would take a date and a timedelta to substitute the resulting datetime in the string.

path: /home/ecmv9074/hpcperm/cmc_data/{date:strftimedelta(-6h;%Y%m%d)}{date:strftimedelta(-6h;%H)}_006

[sandorkertesz]

@chebertpinard, can I ask you how/where you actually want use this pattern? In the "file-pattern" source?

[chebertpinard]

Sorry I meant to do a PR not an issue (#606). This implementation follows a suggestion from @b8raoult.

[sandorkertesz]

Fixed in #606