feat: add load_parameters as a library function

[BoyuanFeng]

Summary

Add load_parameters as a library function in manta-pay such that this function can be used by other Manta repos.

Details

Functions similar to load_parameters have been repeatedly implemented in several Manta repos, such as Manta, manta-signer, and cli. In this PR, we move load_parameters as a library function in manta-pay to remove code redundancy.

Issue: #100

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