Add an utility function to get the first timestamp of a slot (#5316)

# Description

Add `starting_timestamp` function for `Slot` type.

## Integration

This is an addition of public function to a type, so integration should
be seamless for idiomatic use of Rust.

## Review Notes

Since `Slot` is just a slot number, the it's starting timestamp depends
on `SlotDuration` which is a parameter to the added function. This
function can be seen as dual to existing `fn from_timestamp`.
Because there is a potential for overflow, the return type is `Option`.

Q1: should I introduce tests for in this crate and add cases for both
case: overflow (`None`) and no overflow (`Some`)?

Q2: How can I add labels? IMO they should be `T0-node` and `D0-easy` but
I cannot add them using GH interface.

# Checklist

* [x] My PR includes a detailed description as outlined in the
"Description" and its two subsections above.
* [ ] My PR follows the [labeling requirements](CONTRIBUTING.md#Process)
of this project (at minimum one label for `T`
  required)
* External contributors: ask maintainers to put the right label on your
PR.
* [ ] I have made corresponding changes to the documentation (if
applicable)
* [ ] I have added tests that prove my fix is effective or that my
feature works (if applicable)

---------

Co-authored-by: Squirrel <giles.cope@iohk.io>
Co-authored-by: Davide Galassi <davxy@datawok.net>

prdoc/pr_5316.prdoc

+title: add timestamp function to sp-consensus-slots
+
+doc:
+  - audience: Node Dev
+    description: |
+      Added timestamp function to sp-consensus-slots to get the first timestamp
+      of the given slot.
+
+crates:
+  - name: sp-consensus-slots
+    bump: minor

substrate/primitives/consensus/slots/src/lib.rs

@@ -91,6 +91,13 @@ impl Slot {
 		Slot(timestamp.as_millis() / slot_duration.as_millis())
 	}
 
+	/// Timestamp of the start of the slot.
+	///
+	/// Returns `None` if would overflow for given `SlotDuration`.
+	pub fn timestamp(&self, slot_duration: SlotDuration) -> Option<Timestamp> {
+		slot_duration.as_millis().checked_mul(self.0).map(Timestamp::new)
+	}
+
 	/// Saturating addition.
 	pub fn saturating_add<T: Into<u64>>(self, rhs: T) -> Self {
 		Self(self.0.saturating_add(rhs.into()))