[Feature] Support File Index writing

[zjw1111]

Search before asking

• I searched in the issues and found nothing similar.

Motivation

Paimon C++ currently supports reading File Indexes but does not generate them through the table write path.

Solution

• Align File Index functionality and index format with Java Paimon.
• Design the C++ File Index writer interfaces and integrate File Index generation into the table write path.

Anything else?

This is a sub-issue of #158.

Are you willing to submit a PR?

• I'm willing to submit a PR!