Support vector index for Peewee

[breezewish]

A VectorAdaptor is introduced similar to how we did for SQLAlchemy.

It is possible to utilize Peewee's native index, when we only need one SQL statement to add index. Currently we must execute two SQL statements, so that an adaptor is needed.

An example of being native Peewee index:

class VectorIndex(peewee.SQL):

    table: peewee.Table
    field: peewee.Field
    distance_metric: tidb_vector.DistanceMetric

    def __init__(
        self,
        table: peewee.Model,
        field: peewee.Field,
        distance_metric: tidb_vector.DistanceMetric,
    ):
        self.table = table
        self.field = field
        self.distance_metric = distance_metric
        super(VectorIndex, self).__init__("")

    def __sql__(self, ctx: peewee.Context):
        ctx.literal("ALTER TABLE ")
        ctx.sql(self.table)
        ctx.literal(" ADD VECTOR INDEX ")
        with ctx(parentheses=True):
            with ctx(parentheses=True):
                ctx.literal(self.distance_metric.to_sql_func())
                with ctx(parentheses=True):
                    ctx.sql(self.field)
        return ctx