SheetIndex APIs

[Ellord207]

Context:

My team is working on creating features to facilitate the production of 2d drawings and sheets for users using iTwin.js. To do this, we need to be able to insert Sheet Indexes and their relational elements into the iModel. Since there are no Sheet Index TS implementations, we took the initiative and started creating them ourselves.

I have created an accompanying Discussion Post as a more appropriate place for this discussion.

Description

After discussions, I am intending this PR to include the basic implementations for the Sheet Index. This including backend Classes, common props, Codes, and tests.

After more discussion, future PR(s) will follow to answer the following questions:

• How to ensure a consistent order of the Sheet Index Entries during editing, specifically including when using SheetIndexReferences among other scenarios.
• How to automatically generate and update sheet labels/page numbers during editing.
• Enforcing the 1 SheetReference per Sheet rule.
• And possibly other limitations to ensure consistency across the platform.