Load elements from a blockchain smart contract.
The supported blockchains are: Near mainnet, Near testnet.
If no BlockchainType is specified, the default is Near mainnet.
The Loader uses the Mintbase API to interact with the blockchain.
MB_API_KEY environment variable must be set to use this loader.
The API returns 100 NFTs per request and can be paginated using the
startToken parameter.
If get_all_tokens is set to True, the loader will get all tokens
on the contract. Note that for contracts with a large number of tokens,
this may take a long time (e.g. 10k tokens is 100 requests).
Default value is false for this reason.
The max_execution_time (sec) can be set to limit the execution time
of the loader.
