Summary:
Some DDLs can run as autonomous transactions (i.e., not part of a
transaction block) even if transactional DDL mode
(ysql_yb_ddl_transaction_block_enabled) is on. Currently, the only
example is `CREATE INDEX CONCURRENTLY`.

Autonomous DDL transactions use the kDDL session on the local tserver proxy.
Currently, it is not possible to run an autonomous DDL transaction while a
regular transaction block is active (which uses the kPlain session on the local
tserver proxy). Some of our code relies on this fact. This revision adds
a check to ensure that this assumption is not violated.
Jira: DB-19011

Test Plan: Jenkins

Reviewers: stiwary

Reviewed By: stiwary

Subscribers: yql

Differential Revision: https://phorge.dev.yugabyte.com/D48027