In order to know the full size of a b-tree in disaggregated storage we need to track aggregate block size using the __wt_block_disagg_address_cookie-size field. Essentially a block size will be the size of the base + subsequent deltas. 

In order to track the cumulative size we populate a WT_PAGE_BLOCK_META field. 

This is read in via __block_disagg_read_multiple.

In the write path we check if the block_meta indicates the presence of deltas, if so we add the size from block_meta to the size being written. This is found in __wti_block_disagg_write

A Catch2 unit test may be the only path to testing change.

Because we are changing the meaning of cookie->size we will need to change the version of the cookie, for this see WT_BLOCK_DISAGG_ADDR_VERSION and {}wti_block_disagg_addr_unpack. That function will need to be able to read version 0 cookies, while also reading the new version. There should be no changes to support reading version 0 cookies while writing version 1 cookies. The changes will come later when we support b-tree & database level size tracking.

Scope:

• Update the definition of size in disagg address cookie
• Track delta encoded sizes
• Implement a catch2 unit test to ensure block meta tracking is correct.