Add missing support for Custom email provider in EmailProviderManager

[duedares-rvj]

🔧 Changes

• Add support for Custom email provider in EmailProviderManager

📚 References

• Configure email provider

🔬 Testing

• Performed manual testing to verify functionality.
• Added unit tests for JSON marshalling.

Example

emailProvider: &EmailProvider{
          Name:               auth0.String("custom"),
          Enabled:            auth0.Bool(true),
          DefaultFromAddress: auth0.String("accounts@example.com"),
          Credentials:        &EmailProviderCredentialsCustom{},
}

err := api.EmailProvider.Create(context.Background(), emailProvider)
if err != nil {
    if err.(Error).Status() != http.StatusConflict {
	    t.Error(err)
    }
}

📝 Checklist

• All new/changed/fixed functionality is covered by tests (or N/A)
• I have added documentation for all new/changed functionality (or N/A)

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 95.81%. Comparing base (831f015) to head (55d6bab).

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #452   +/-   ##
=======================================
  Coverage   95.81%   95.81%           
=======================================
  Files          51       51           
  Lines        8456     8461    +5     
=======================================
+ Hits         8102     8107    +5     
  Misses        236      236           
  Partials      118      118           

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[developerkunal]

LGTM