Add generated ETag header when embedding content for caching

[thomasvvugt]

We might want to add this feature when embedding and/or serving static files.
See RFC7232 Section 2.3 and this question on Stack Overflow.

[Fenny]

This might be something for Fiber core also, since fasthttp doesn't seem to set e-tag headers

[arsmn]

I think it should be in the core and/or another middleware for cache controlling

[thomasvvugt]

@Fenny Please move this issue to gofiber/fiber repo for future work on implementing ETags

[Fenny]

gofiber/fiber#311