Fix/viper race condition

[ljluestc]

Fix: race condition in viper config handling

Description

This PR fixes a race condition in the configs package where viper.WatchConfig updates the configuration concurrently with read operations.
It introduces a sync.RWMutex to protect access to the global config variable.

Issue

Fixes #108

Type of Change

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Refactor (no functional changes, no api changes)
• Documentation
• Test

Checklist

• I have performed a self-review of my own code
• I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes
• I have commented my code, particularly in hard-to-understand areas

Verification

Run the race detector test:

go test -v -race ./configs/...

Output:

=== RUN   TestRace
--- PASS: TestRace (0.05s)
PASS
ok      github.com/xinliangnote/go-gin-api/configs      1.065s