feat: support ISO format for dates in the barman-cloud output

[mnencia]

Closes #48

[jsilvela]

Looks good.
One comment is that we could have a begin time in ISO format, and an end time in non-ISO format, and the code would work fine.
Would it make sense to add a consistency check? Either we use ISO format for both begin/end, or for neither.
In any case, optional.

    if b.EndTimeISOString != "" {
         ... snipped ...
    } else if b.EndTimeString != "" {

[mnencia]

Looks good. One comment is that we could have a begin time in ISO format, and an end time in non-ISO format, and the code would work fine. Would it make sense to add a consistency check? Either we use ISO format for both begin/end, or for neither. In any case, optional.

    if b.EndTimeISOString != "" {
         ... snipped ...
    } else if b.EndTimeString != "" {

If we add such a check, what is the desired behavior in case of inconsistency? I cannot imagine a situation where it happens, given that both the values are produced in a single berman-cloud-backup execution, so they are consistent by definition.