Since we want to remove the requirement of disallowing "explicitly disabled"
CDC table when creating external index (#29894), we still need to check other
CDC required parameters to be set properly.
Before this commit, once we auto-enable CDC which was "explicitly disabled",
we would never run the `check_cdc_options()`.
This patch adjusts the check to happen not only when the CDC enabled is true.
Fulltext indexes rely on CDC to track changes for asynchronous index
building. Enforce the following CDC constraints during CREATE INDEX:
- CDC TTL must be at least 86400 seconds (24 hours)
- CDC delta mode must be 'full' or postimage must be enabled
Add `has_fulltext_index()` and `check_cdc_options()` so that other
modules can detect fulltext indexes and validate CDC settings:
- include fulltext indexes in `cdc_enabled()` so the CDC log
is auto-created, and validate CDC options in
`on_before_update_column_family()`
- block `ALTER TABLE ... WITH cdc = {'enabled': false}`
when a fulltext index exists on the table
Add `external_index` as a common base for `vector_index` and `fulltext_index`,
both of which are backed by an external Vector Store engine and share CDC
requirements.
