ICU-23156 Implement icupkg --keep option

[dzfrias]

The icupkg --keep option allows users to only keep a certain subset of data items within the data file, and discard the rest. This is to support the workflow of:

1. Profiling the data items accessed via utrace
2. Only keeping the relevant items in the data file

This is a workflow that I would find personally helpful in my work to downsize the icudt.dat that I'm currently working on.

Checklist

• Required: Issue filed: ICU-23156
• Required: The PR title must be prefixed with a JIRA Issue number. Example: "ICU-1234 Fix xyz"
• Required: Each commit message must be prefixed with a JIRA Issue number. Example: "ICU-1234 Fix xyz"
• Issue accepted (done by Technical Committee after discussion)
• Tests included, if applicable
• API docs and/or User Guide docs changed or added, if applicable

[dzfrias]

Is there any progress I should make on this PR? Tests?