Implement AirdropsDatabaseReadableKVState

[IvanKavaldzhiev]

Problem

We need to add a new type of database accessor to read airdrop related data, so that we have specific ReadableKVState instance for this type of data.

Solution

Add AirdropsDatabaseReadableKVState class that reads airdrop data and converts it to AccountPendingAirdrop PBJ type.

Add unit tests to cover all cases including historical reads and correct mapping for all fields.

Alternatives

No response