Summary:
# Background

Postgres stores everything in 8K pages. What happens when you have a large value (e.g. a long string) that is longer than 8K? Postgres uses TOAST to deal with this. TOAST (The Oversized Attribute Storage Technique) has two approaches that it uses to fit large tuples into 8K pages.
1. Compression: Postgres always tries this first--it compresses the datums in the tuple one by one until either the target size has been reached or everything has been compressed and it's still too big.
2. Out-of-line storage—for each table PostgreSQL also creates an auxiliary TOAST table for values that are still >8160B after compression. It splits the value up into 2K chunks and stores them in the TOAST table. Then, in the main table, it stores a pointer to the TOAST table entry.

# The state of toasting in YugabyteDB

For regular tables, we generally have all forms of toasting disabled. This is mainly because we replaced Postgres's page-based storage engine with our own RocksDB-based storage engine, which does not have the page size constraint (RocksDB also applies its own compression to SST files on disk, but it decompresses them as soon as it reads them into the in-memory block cache).

We recently enabled toast compression for catalog tuples in the catcache. This only affects the catcache and does not affect how they are stored in DocDB. Toast tables remain disabled for all DocDB tables. See {D29916}.

# Temp tables

Temp tables in YB use Postgres's storage engine rather than DocDB. This means that they have the 8K size restriction, and, because we don't support toast tables at all, large values cannot be inserted into temp tables. Attempts to insert such values will result in an error such as the following:
```
row is too big: size 8560, maximum size 8160
```

# The fix

To fix this, we enable toast tables for temp relations. These toast tables are themselves also considered temporary and use PG storage. They have all the other properties of temp tables (dropped at the end of a session, etc.)
Jira: DB-15339

Test Plan:
```
./yb_build.sh release --cxx-test pg_temp_toast-test
```

The data generation in this test has been modified to force Postgres to use both compression and out-of-line storage for the rows we insert into PG.

Reviewers: fizaa

Reviewed By: fizaa

Subscribers: yql

Differential Revision: https://phorge.dev.yugabyte.com/D42530