Bundle script and entrypoints for ContractAbstraction instantiation

[roxaneletourneau]

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When creating an instance of ContractAbstraction, the at method of the Contract and Wallet API, does calls to the RPC to fetch the script and the entrypoints of the contract. However, the script and entrypoints are statics values for a contract and Taquito should provide a way for the developer user of Taquito to specify them and thus avoid making calls to the RPC to fetch their values.

Relates to: #917

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Implement a new class ContractsLibrary which can be populated by users with contract addresses and their corresponding script and entrypoints.
The user will inject the contractLibrary instance to the TezosToolkit as an extension using its addExtension method.
If a contractLibrary is set, the at method of the Contract or Wallet API will fetch contract values from it otherwise, the values will be fetched from the RPC as usual.