Add Custom Message Ability

[LRFalk01]

Thanks for this module!

I'm curious what the interest level is for the ability to customize the log message itself.

Adding a config such as the following would convert the default log message of Incoming request to something more like this: [200] GET /request/path

WithCustomMessage: func(c *gin.Context) string {
	return fmt.Sprintf(
		"[%d] %s %s",
		c.Writer.Status(),
		c.Request.Method,
		c.Request.URL.Path,
	)
},

[codecov]

Codecov Report

❌ Patch coverage is 0% with 13 lines in your changes missing coverage. Please review.
✅ Project coverage is 0.00%. Comparing base (2e0bde9) to head (49f7a5d).
⚠️ Report is 2 commits behind head on main.

Files with missing lines	Patch %	Lines
middleware.go	0.00%	13 Missing ⚠️

Additional details and impacted files

@@          Coverage Diff          @@
##            main     #77   +/-   ##
=====================================
  Coverage   0.00%   0.00%           
=====================================
  Files          4       4           
  Lines        372     377    +5     
=====================================
- Misses       372     377    +5     

Flag	Coverage Δ
unittests	0.00% <0.00%> (ø)

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