feat(sdk): LinkedChunk can hold a value for Gaps.

This patch updates `ChunkContent::Gap` to hold a content `U`. Thus,
`Chunk` and LinkedChunk` both get a new generic parameter `U`. Some
methods like `new_gap` or `insert_gap_at` take a new `content: U`
parameter.

This type `RoomEvents` (that uses `LinkedChunk`) is also updated
accordingly.

Author: Hywan