Public MASP frontend fee builder

[grarco]

Describe your changes

Makes the compute_masp_frontend_sus_fee function of the SDK public for external (Namadillo) usage. Also, updates this function to account for the remainder amount.

Checklist before merging

• If this PR has some consensus breaking changes, I added the corresponding breaking:: labels
  • This will require 2 reviewers to approve the changes
• If this PR requires changes to the docs or specs, a corresponding PR is opened in the namada-docs repo
  • Relevant PR if applies:
• If this PR affects services such as namada-indexer or namada-masp-indexer, a corresponding PR is opened in that repo
  • Relevant PR if applies:

[mergify]

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