refactor: Remove client/keys.KeysCdc

[jacobhjkim]

Description

Closes: #8047

Replace client/keys.KeysCdc with encoding/json.

• I have tested CLI JSON outputs between client/keys.KeysCdc and encoding/json. The output doesn't seem to change at all.
• Deleted unused client.keys/codec.go and client.keys/codec_test.go.

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Author Checklist

All items are required. Please add a note to the item if the item is not applicable and
please add links to any relevant follow up issues.

I have...

• included the correct type prefix in the PR title
• added ! to the type prefix if API or client breaking change
• targeted the correct branch (see PR Targeting)
• provided a link to the relevant issue or specification
• followed the guidelines for building modules
• included the necessary unit and integration tests
• added a changelog entry to CHANGELOG.md
• included comments for documenting Go code
• updated the relevant documentation or specification
• reviewed "Files changed" and left comments if necessary
• confirmed all CI checks have passed

Reviewers Checklist

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your handle next to the items reviewed if you only reviewed selected items.

I have...

• confirmed the correct type prefix in the PR title
• confirmed ! in the type prefix if API or client breaking change
• confirmed all author checklist items have been addressed
• reviewed state machine logic
• reviewed API design and naming
• reviewed documentation is accurate
• reviewed tests and test coverage
• manually tested (if applicable)

[amaury1093]

utACK, reviewed all the structs where we switched from amino to std json, and their outputs should be the same

[codecov]

Codecov Report

Merging #13781 (99fbbf6) into main (22f3261) will decrease coverage by 0.00%.
The diff coverage is 0.00%.

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #13781      +/-   ##
==========================================
- Coverage   56.71%   56.70%   -0.01%     
==========================================
  Files         636      635       -1     
  Lines       54556    54547       -9     
==========================================
- Hits        30940    30932       -8     
+ Misses      21110    21109       -1     
  Partials     2506     2506              

Impacted Files	Coverage Δ
client/keys/add.go	63.34% <0.00%> (ø)
client/keys/parse.go	77.01% <0.00%> (ø)
client/keys/utils.go	27.08% <0.00%> (+0.55%)	⬆️