[Feature] SaveAsByTemplateI: Make SaveAsByTemplateImplAsync with Stream parameter public

[Ashymonth]

Description:
Currently, there are two public methods that accept different input types (string path and byte[]) for template data, both of which internally convert to a Stream and call SaveAsByTemplateImplAsync. However, the Stream-based implementation is marked as internal.

This creates unnecessary limitations when:

The template data is already available as a Stream

The source isn't a file path or byte array (e.g., from network, in-memory generated stream, etc.)

Request:
Please make SaveAsByTemplateImplAsync public so we can directly pass Stream objects. This would:

1. Provide more flexibility in how template data is provided
2. Avoid unnecessary conversions
3. Maintain consistency with common .NET patterns where Stream-based methods are publicly available

Example usage:

using var stream = GetTemplateFromSomewhere();
await document.SaveAsByTemplateAsync(stream, data);

Current workaround:
Users must either:

Write to a temporary file and use the path-based version, or

Convert to byte array and use that version

Both workarounds have performance and resource implications.