conditional ddl support; add autogeneration support for `ddf_if()`

Describe the bug

Using conditional Indices with ddl_if, alembic ignores the target dialect and creates all the indices, not just the index that matches the conditional constraint.

from sqlalchemy import Index, String
from sqlalchemy.orm import DeclarativeBase, Mapped, mapped_column


class Base(DeclarativeBase):
    pass


class User(Base):
    __tablename__ = "user"

    __table_args__ = (
        Index("my_pg_index_psql", "name").ddl_if(dialect="postgresql"),
        Index("my_pg_index_sqlite", "name").ddl_if(dialect="sqlite"),
    )

    id: Mapped[int] = mapped_column(primary_key=True)
    name: Mapped[str] = mapped_column(String(30))

./bin/alembic revision --autogenerate -m 'DB init'
INFO  [alembic.runtime.migration] Context impl PostgresqlImpl.
INFO  [alembic.runtime.migration] Will assume transactional DDL.
INFO  [alembic.autogenerate.compare] Detected added table 'user_account'
INFO  [alembic.autogenerate.compare] Detected added index ''my_pg_index_psql'' on '('name',)'
INFO  [alembic.autogenerate.compare] Detected added index ''my_pg_index_sqlite'' on '('name',)'

Generate python file contains two create_index statements, should be a single stmt:

def upgrade() -> None:
    # ### commands auto generated by Alembic - please adjust! ###
    op.create_table('user',
    sa.Column('id', sa.Integer(), nullable=False),
    sa.Column('name', sa.String(length=30), nullable=False),
    sa.PrimaryKeyConstraint('id')
    )
    op.create_index('my_pg_index_psql', 'user_account', ['name'], unique=False)
    op.create_index('my_pg_index_sqlite', 'user_account', ['name'], unique=False)
    # ### end Alembic commands ###

Expected behavior
Only the index for the target dialect should be created, not both. Note if you use the same index name for both Index defs, you will get a single op.create_index but the configuration will be random.

To Reproduce
As above, using postgres as the db in alembic.ini:

sqlalchemy.url = postgresql://postgres:password@localhost/postgres

Versions.

• OS: Linux
• Python: 3.12.2
• Alembic: 1.13.1
• SQLAlchemy: 2.0.31
• Database: Postgres 14
• DBAPI: ?

Start with Alembic's autogenerate index comparison and the SQLAlchemy Index.ddl_if() behavior, using the provided PostgreSQL reproduction and alembic.ini configuration. Done means the generated migration contains only the index whose conditional dialect matches the target database, rather than both op.create_index statements.