FOCUS #617: FOCUS dataset - glossary term update

[ijurica]

This PR updates the FOCUS specification to clarify the inclusion of custom/native provider-specific columns (prefixed with x_) in the FOCUS dataset.

Changes:

1. Glossary Update:

   • Updated the FOCUS dataset glossary term to specify that x_ columns should be included when they provide additional information not captured by existing FOCUS columns.
   • Clarified that introducing custom columns must adhere to aggregation-related requirements for metric columns (e.g., costs, quantities) to prevent aggregation errors.

2. Introduction/Appendix Addition (pending discussion):

   • Potential addition of explicit mentions in the Introduction and/or a new appendix to emphasize that x_ columns containing essential provider-specific information should also be included in the dataset.

[bahung]

I think we can add some reference (anonymized) data and related links to the CSPs' docs on their FOCUS data exports. Let me know if I can be of help.