Format: Include small sample of the data to the HATS metadata

[hombit]

Feature request

It would be really helpful to have a small portion of the data as a catalog metadata. Main reasons for that are:

1. Good for data exploration, so users could have a better idea of the data schema and units, especially in the case of lack of column descriptions
2. It would really help with some technical tasks, for example with meta derivation for user-defined functions.

This data sample could come in different ways, for example as a catalog "head" or a single row from each partition. This data may go to a new metadata file, like sample.parquet, or replace _common_metadata file.

Before submitting
Please check the following:

• I have described the purpose of the suggested change, specifying what I need the enhancement to accomplish, i.e. what problem it solves.
• I have included any relevant links, screenshots, environment information, and data relevant to implementing the requested feature, as well as pseudocode for how I want to access the new functionality.
• If I have ideas for how the new feature could be implemented, I have provided explanations and/or pseudocode and/or task lists for the steps.