Is it possible in SPFX to make a REST API call with a service account instead of the current users’ credentials?
This is not possible from within the SharePoint Framework. REST calls execute on behalf of the current user. If you need to use another credential, you’ll have to create a server side component such as your own WebAPI or using an Azure Function HTTP trigger that will allow you to execute on behalf of another user.
You can call an external API from SPFx which is a recent possibility using the AAD authentication. Check out this link https://developer.microsoft.com/en-us/office/blogs/calling-external-apis-securely-from-sharepoint-framework/
But this is going to be an API call & anyone who have authenticated in to your AD will be able to call this API. You need to manage your security within your API layer.
