fix: use pydantic models for create index payloads

[PopescuTudor]

No description provided.

[radu-mocanu]

why don t we create a model for this and add it as an optional member of DataSourceBase?

class Indexer(BaseModel):
     model_config = ConfigDict(alias_generator=to_camel, populate_by_name=True)

   cron_expression: str
   time_zone_id: str
   
   @model_validator(mode = 'before')
   def validate():
     ... #we can even add a cron expression validator here 

class BaseSourceConfig(BaseModel):
   indexer: Optional[Indexer] = None
   ...

[radu-mocanu]

let s use the DataSourceBase here