Implement standardized country flag data for mentor profiles

[Madhawa97]

Description:
Update the backend to support standardized country selection and storage for mentor profiles.

Tasks:

• Update the database schema (if necessary)
• Implement API endpoint for retrieving the list of countries
• Implement server-side validation for country data (if necessary)

Acceptance Criteria:

• Create an API endpoint to serve a list of valid countries (e.g., /api/countries)
• Implement server-side validation to ensure only valid country codes are stored
• Update relevant queries to include country information where appropriate
• Ensure proper error handling for invalid country data

Additional Information:

Related Dependencies or References:
Related frontend issue: sef-global/scholarx-frontend#165

[Disura-Randunu]

@Madhawa97 @anjula-sack Shall I work on this one?

[anjula-sack]

Go ahead @Disura-Randunu

[Disura-Randunu]

@anjula-sack shall I proceed with the mentor changes for country on this same issue?

[anjula-sack]

@anjula-sack shall I proceed with the mentor changes for country on this same issue?

yeah sure.