feat: update config service for template handling + add structure join

src/configuration/config.dev.ts

src/configuration/config.prod.ts

src/configuration/config.ts

src/configuration/configuration.service.ts

src/mailer/mail-templates/structureJoinRequest.ejs

src/mailer/mail-templates/structureJoinRequest.json

src/structures/services/structures.service.ts

src/structures/structures.controller.ts

src/users/users.service.ts