Support allocating memory for large strings

PostgreSQL's MaxAllocSize limit prevents storing individual variable-length character strings exceeding ~1GB, causing
"invalidmemory alloc request size" errors during INSERT operations on tables with large text columns. Example
reproductionincluded in artifacts.md. 

This limitation also affects pg_dump when exporting a PostgreSQL database with such data. The attached patches
demonstratesa proof of concept using palloc_extended with MCXT_ALLOC_HUGE in the write path. For the read path, there
area couple of possible approaches: extending existing functions to handle huge allocations, or implementing a chunked
storagemechanism that avoids single large allocations. 

Thoughts?

Maxim