[BUG]: Beanie BackLink does not support multiple types

[mike-pisman]

It is impossible to use multiple types with Beanie BackLink, as a result, the document stores the whole linked document. For example:

class Policy(Document):
    parent_resource: BackLink["Workspace"] | BackLink["Group"] | BackLink["Poll"] = Field(original_field="policies")

When creating new Policy, the parent_resource field stores the whole Workspace/Group document instead, which is undesirable.

The underlying issue has been mentioned in BeanieODM/beanie#746

[mike-pisman]

A workaround for this issue:

the parent_resource was converted to the Link type, which is manually populated using create_link() method. Since Beanie cannot/struggles to fetch links of multiple types, two new class methods were added: get_parent_resource() uses Link reference to find and return parent document(Workspace/Group/Poll), and get_policy_holder() finds and returns document of policy holder(Account/Group).

For more info, see #76