Add more filters to List group milestones

[adamcohen2]

This MR adds the following filters to the List group milestones endpoint:

• SearchTitle
• IncludeAncestors
• IncludeDescendents
• UpdatedBefore
• UpdatedAfter
• ContainingDate
• StartDate
• EndDate

See Document fields for List Group milestones API for more details.

[adamcohen2]

The newly added*time.Time fields should be *ISOTime fields (using the ISO 8601 format) instead. If you can update that, this PR is good to go...

@svanharmelen done, thanks for the quick review!