Support for cyclic SO references

[Dosant]

In scope of this issue we need to make sure that features that rely on saved object references support cyclic references.

Cyclic reference use-case came up in: #71409 (comment), where dashboards can link to each other via drilldowns.

From a quick poc it became clear that saved object reference system isn't implemented or wasn't tested to gracefully support cyclic references.

Example setup:

• Dashboard A is linking to Dashboard B via dashboard drilldown
• Dashboard B is linking to Dashboard A via dashboard drilldown

SO Relations UI:

(Looks good)

Export Dashboard A with related objects:

Error:

{"statusCode":400,"error":"Bad Request","message":"circular reference: [dashboard:09599400-0fa8-11eb-9974-3fd3df6ee7d4] ref-> [dashboard:f61a0eb0-0fa7-11eb-9974-3fd3df6ee7d4] ref-> [dashboard:09599400-0fa8-11eb-9974-3fd3df6ee7d4]"}

Copy Dashboard A to different space:

Error:

{"statusCode":500,"error":"Internal Server Error","message":"An internal server error occurred."}

TODO: check other features that rely on references (no sure what else...)

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Needed for #71409

[elasticmachine]

Pinging @elastic/kibana-platform (Team:Platform)