Allow the shielded reward precision for tokens to be explicitly set.

[murisi]

Describe your changes

An attempt to address #4422 by introducing new storage keys to hold the shielded reward precision for each token and reading that instead of deducting it. More precisely, the changes are as follows:

• Made a new storage key to hold the shielded rewards precision for each token
• The function for obtaining a token's reward precision now reads this new storage key
  • And if the storage key is uninitialized, it initializes it using the old algorithm: 10^(max(denom,3)-3)

This PR should be backwards compatible in the sense it can read/support current mainnet state without any migrations. And simultaneously it allows the shielded rewards precision to be set for new tokens as they are introduced by governance proposals. But to fix the precision of IBC tokens that have already been introduced into the system, purging of its ConversionLeaf::conversion entries (by setting them to MaspAmount::zero()) would also have to be done in addition to setting its desired shielded reward precision in storage. (More generally, once the shielded reward precision for a token is set, it cannot be changed without doing a similar purging process.)

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:

[codecov]

Codecov Report

Attention: Patch coverage is 90.62500% with 3 lines in your changes missing coverage. Please review.

Project coverage is 74.49%. Comparing base (7ca07c9) to head (35b1045).
Report is 30 commits behind head on main.

Files with missing lines	Patch %	Lines
crates/token/src/lib.rs	0.00%	3 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #4423      +/-   ##
==========================================
- Coverage   74.53%   74.49%   -0.05%     
==========================================
  Files         339      339              
  Lines      110504   110653     +149     
==========================================
+ Hits        82368    82433      +65     
- Misses      28136    28220      +84     

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[yito88]

@murisi Do we need to modify the e2e test ibc_token_inflation to check this change is effective?

[murisi]

@murisi Do we need to modify the e2e test ibc_token_inflation to check this change is effective?

Thanks, good point. I've introduced an integration test named masp::dynamic_precision to test this behaviour since that's easier (and probably faster) to run than an E2E test, and because this change this change is not IBC specific.