Summary:
Adding a vector column with a default value breaks some of the assumptions of our vector
implementation. The in-progress work to have the reverse vector mapping owned by the table with the
vector column means a reverse mapping entry must be written when a vector column is
inserted. However, when adding a vector column to an existing table with a default value, no docdb
inserts are executed. So there is a question of when to create vector column values using the
default value and a generated vector id for the backfill.

This diff solves this problem by rejecting any DDL to add a vector column to an existing table with
a default value.

Test Plan:
```
./yb_build.sh --cxx-test pg_vector-test --cxx-test-filter-re pg_vector-test --gtest_filter 'PgVectorTest.RejectAddVectorColumnWithDefault'
```

Reviewers: sergei, arybochkin, myang

Reviewed By: arybochkin, myang

Subscribers: ybase, yql

Differential Revision: https://phorge.dev.yugabyte.com/D55818