fix: add explicit String path overloads

[claymcleod]

Added explicit String overloads for basename and size to remove the overload ambiguity introduced when WDL 1.2 added Directory. Expanded the size example to cover required and optional string paths. Closes #683.

This targeted wdl-1.2, the earliest affected minor version. The change must be forward-ported to wdl-1.3 and wdl-1.4 after it merges.

Before submitting this PR, please make sure:

• You have added a few sentences describing the PR here.
• You have considered whether the README.md or other documentation needs updating to account for these changes.
• You have updated the CHANGELOG.md describing the change and linking back to your pull request.
• You have read and agree to the CONTRIBUTING.md document.
• You have considered adding or updating relevant example WDL tests to the specification.
  • See the guide for more details.

For OpenWDL team members:

• Assign the appropriate individual to this PR.
• Triage the PR and add appropriate labels.