Add pragma

[sisyphusSmiling]

Closes: #7

Relevant: onflow/cadence#2353 & onflow/cadence#2380

Description

Cadence recently introduced an account linking pragma #allowAccountLinking as an interim security measure. Currently, the pragma check is done statically, and must be stated statically in the contract in which linkAccount() is called. This means that the ChildAccount contract is currently broken on Testnet.

This PR adds the account linking pragma to ChildAccount contract and account linking transactions to enable AuthAccount.linkAccount().

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