i18n/gi18n:Add GetAllContent Method to i18n Component

[Jerry-fmcheng]

Is your feature request related to a problem?

Option No

Describe the solution you'd like

Problem Statement
Currently, the i18n component in GoFrame provides the GetContent method, which retrieves the translation for a specific key. However, there is no built-in method to retrieve all translations for a given language. This functionality is essential in scenarios where developers need to:

Export all translations for a specific language.
Debug or validate the completeness of translations.
Programmatically process or manipulate all translations.

Proposed Solution
Add a GetAllContent method to the i18n component, which would return all translations for a specified language as a map[string]string (where the key is the translation key and the value is the translated content).

Proposed Method Signature
func (m *Manager) GetAllContent(ctx context.Context) (map[string]string)

Describe alternatives you've considered

This feature would be backward-compatible and would not impact existing functionality

Additional

No response