Add polymorphic deserialization for the GHAppInstallation account field

[anujhydrabadi]

Description

Fixes #1497
Recovering PR #1522
There is a problem: this works fine for events, but when deserializing App Installation GET API responses, the type field (based on which we identify whether the account is User or Org) gets unset to null. The account polymorphic deserialization part works fine though. Perhaps there is a difference in how we are using Jackson at both places (events vs regular API responses), but I have not been able to figure it out yet.

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