Use resource strings in IdentityApiEndpointRouteBuilderExtensions for localization

[halter73]

          I assume we'll want to localize these strings in the future?

Originally posted by @BrennanConroy in #49498 (comment)

[hishamco]

@halter73 In such a case can I start the PR immediately because there are no proposed APIs or do you follow a specific flow?

[halter73]

Were you just planning on moving strings unchanged into a Resources.resx file? If so, I say go for it.

[hishamco]

What if we want to add localized strings?