feat: Decouple staking logic from the tier module

[iverc]

Description

This PR streamlines the Tier module by removing all interactions with the staking keeper (e.g., Delegate, Undelegate, BeginRedelegation, etc.) and introduces several fixes and improvements.

As a result, the developer stake will remain stored in the Tier module, so that it is no longer affected by slashing and won't be eligible for rewards.

Tasks

• Refactor Lock, Unlock, Redelegate, and CancelUnlocking.
• Get rid of unbondTime and only rely on unlockTime to complete the unlocking.
• Return error if no unlocking lockup found in SubtractUnlockingLockup.
• Expect unlockTime and completionTime to be nil instead of time.Time{} to reduce ambiguity.
• Add creation_height to MsgUnlockResponse.
• Refactor proratedCredit and add Test_CalculateProratedCredit.
• Update and fix corresponding tests.

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 96.42857% with 3 lines in your changes missing coverage. Please review.

Project coverage is 27.98%. Comparing base (24c0f5c) to head (13ca3fe).

Files with missing lines	Patch %	Lines
x/tier/keeper/keeper.go	93.10%	1 Missing and 1 partial ⚠️
x/tier/keeper/lockup.go	96.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##              dev      #11      +/-   ##
==========================================
- Coverage   28.53%   27.98%   -0.55%     
==========================================
  Files         148      148              
  Lines        6225     6128      -97     
==========================================
- Hits         1776     1715      -61     
+ Misses       4336     4312      -24     
+ Partials      113      101      -12     

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