[Feature] Add support for database policies with relations

DAB currently offers support for simple Odata expressions when defining “RLS” policies for an entity as explained in the [public doc](https://learn.microsoft.com/en-us/azure/data-api-builder/reference-configuration?tabs=http#database).

```
{
"entities": {
"User": {
"source": {
"object": "dbo.Users",
"type": "table"
},
"permissions": [
{
"role": "selfReader",
"actions": [
{
"action": "read",
"policy": {
"database": "@claims.userId eq @item.Id"
}
}
]
}
]
}
}
}
```

This instructs DAB to return rows from User table only for which Id value matches with the userId claim. The expression supports filtering on columns on the same entity only.

In many cases, customers keep user details or mapping details on a separate table altogether. Consider the following database schema.

Books
-Id
-Title
-SellerId (relationship with Sellers on its id)

Sellers
-id
-email

Let’s say we want to return Book rows only if its seller email is the same as caller’s email.
To support his behavior, we would need support for enhanced Odata expressions such as
`@item.sellerId/all(s: s/email eq @claims.upn)`
OR 
`@item.sellerId/any(s: s/email eq @claims.upn)`