Audit or add validation to rest endpoints in `synapse/rest/client/filter.py`

This issue has been migrated from [#14265](https://github.com/matrix-org/synapse/issues/14265).

---

Only one endpoint here: CreateFilterRestServlet's POST

(there's a GET endpoint but there's not much to validate there)

We have a JSONSchema for filter definitions. Could try using https://pydantic-docs.helpmanual.io/datamodel_code_generator/ to generate the model rather than doing it from first principles.

```
python -c 'from synapse.api.filtering import USER_FILTER_SCHEMA; import json; print(json.dumps(USER_FILTER_SCHEMA))' | datamodel-codegen --input-file-type jsonschema --strict-types str int bool float
```

```
# generated by datamodel-codegen:
# filename:
# timestamp: 2022-10-21T21:37:20+00:00

from __future__ import annotations

from enum import Enum
from typing import List, Optional

from pydantic import BaseModel, Extra, Field, StrictBool, StrictFloat, StrictStr, constr

class EventFormat(Enum):
client = 'client'
federation = 'federation'

class RoomIdArray(BaseModel):
__root__: List[StrictStr]

class UserIdArray(BaseModel):
__root__: List[StrictStr]

class Filter(BaseModel):
class Config:
extra = Extra.forbid

limit: Optional[StrictFloat] = None
senders: Optional[UserIdArray] = None
not_senders: Optional[UserIdArray] = None
types: Optional[List[StrictStr]] = None
not_types: Optional[List[StrictStr]] = None
org_matrix_msc3874_rel_types: Optional[List[StrictStr]] = Field(
None, alias='org.matrix.msc3874.rel_types'
)
org_matrix_msc3874_not_rel_types: Optional[List[StrictStr]] = Field(
None, alias='org.matrix.msc3874.not_rel_types'
)

class RoomEventFilter(BaseModel):
class Config:
extra = Extra.forbid

limit: Optional[StrictFloat] = None
senders: Optional[UserIdArray] = None
not_senders: Optional[UserIdArray] = None
types: Optional[List[StrictStr]] = None
not_types: Optional[List[StrictStr]] = None
rooms: Optional[RoomIdArray] = None
not_rooms: Optional[RoomIdArray] = None
contains_url: Optional[StrictBool] = None
lazy_load_members: Optional[StrictBool] = None
include_redundant_members: Optional[StrictBool] = None
unread_thread_notifications: Optional[StrictBool] = None
org_matrix_msc3773_unread_thread_notifications: Optional[StrictBool] = Field(
None, alias='org.matrix.msc3773.unread_thread_notifications'
)
org_matrix_labels: Optional[List[StrictStr]] = Field(
None, alias='org.matrix.labels'
)
org_matrix_not_labels: Optional[List[StrictStr]] = Field(
None, alias='org.matrix.not_labels'
)
related_by_senders: Optional[List[StrictStr]] = None
related_by_rel_types: Optional[List[StrictStr]] = None

class RoomFilter(BaseModel):
class Config:
extra = Extra.forbid

not_rooms: Optional[RoomIdArray] = None
rooms: Optional[RoomIdArray] = None
ephemeral: Optional[RoomEventFilter] = None
include_leave: Optional[StrictBool] = None
state: Optional[RoomEventFilter] = None
timeline: Optional[RoomEventFilter] = None
account_data: Optional[RoomEventFilter] = None

class Model(BaseModel):
class Config:
extra = Extra.forbid

presence: Optional[Filter] = None
account_data: Optional[Filter] = None
room: Optional[RoomFilter] = None
event_format: Optional[EventFormat] = None
event_fields: Optional[List[constr(regex=r'^((?!\\\\).)*$', strict=True)]] = None

```