[flake8-tidy-imports] Add `extend-banned-api` (#28644)
## Summary
Add `lint.flake8-tidy-imports.extend-banned-api` so we can add banned
APIs without repeating an inherited `banned-api` table. Entries in
`extend-banned-api` override entries with the same key in `banned-api`,
including their diagnostic messages.
For example, a child `ruff.toml` can add a ban while retaining the bans
from its base configuration:
```toml
extend = "../ruff.toml"
[lint.flake8-tidy-imports.extend-banned-api]
"typing.TypedDict".msg = "Use typing_extensions.TypedDict instead."
```
This follows the existing `extend-aliases` behavior: the base and
extension tables inherit independently, and we merge them after
resolving configuration inheritance. A child can replace either table,
or clear it with `{}`. The change includes option documentation, the
generated JSON schema, and CLI coverage for standalone bans,
inheritance, message overrides, and clearing either table.
