Rename IssuanceKey struct to IssuanceMasterKey

[vivek-arte]

This PR makes a consistent rename of the struct in order to improve consistency with ZIP 227.

[what-the-diff]

PR Summary

This update has one main change discussed as follows:

• Updating Key Type from IssuanceKey to IssuanceMasterKey

The PR introduces a term change from IssuanceKey to IssuanceMasterKey. This update can be found throughout several files. The new term IssuanceMasterKey appears clearer and more descriptive, potentially leading to better user understanding when navigating through the code.

The files affected by this change include:

• burn_validation.rs: This term was changed in a function get_burn_tuple.
• issuance.rs: The change was implemented multiple times in this file.
• keys.rs: The old term was completely replaced with the new term.
• note/asset_base.rs: The term was changed in a function random.
• supply_info.rs: This term was modified in a function create_test_asset.
• zsa.rs: This term was changed in a function prepare_keys.

The change enhances readability, making the code easier to follow and understand.

[vivek-arte]

Closed in favour of #92