Update generated index.pidx to become compliant with the schema

[jkrech]

Describe the bug
The index.pidx. generated by vidx2pidx fail the schema check against:
https://raw.githubusercontent.com/Open-CMSIS-Pack/Open-CMSIS-Pack-Spec/main/schema/PackIndex.xsd

To Reproduce
Steps to reproduce the behavior:

1. Run the vidx2pidx tool
2. Run a schema check against PackIndex.xsd
3. missing mandatory elements:

• <vendor> = take the filename of the file passed as parameter "vendors.vidx" -> "vendors" (BTW this should be singular "vendor" in the help/usage information.
• <url> = take the absolute path of the generated "index.pidx" (or whatever is specified by -o )
• <timestamp> = take the dateTime where hours, minutes is sufficient e.g.: 2024-08-20T04:00:20

Expected behavior
vidx2pidx must generate schema compliant index.pidx

Environment (please complete the following information):

• OS: all of windows/linux/darwin
• Version latest