Add policy expiration (#616)

#593

VM PR : FuelLabs/fuel-vm#871

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- [x] I have reviewed the code myself

### After merging, notify other teams

[Add or remove entries as needed]

- [ ] [Rust SDK](https://github.com/FuelLabs/fuels-rs/)
- [ ] [Sway compiler](https://github.com/FuelLabs/sway/)
- [ ] [Platform
documentation](https://github.com/FuelLabs/devrel-requests/issues/new?assignees=&labels=new+request&projects=&template=NEW-REQUEST.yml&title=%5BRequest%5D%3A+)
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- [ ] Someone else?

src/tx-format/policy.md

@@ -7,12 +7,13 @@ enum PolicyType : uint32 {
     WitnessLimit = 2,
     Maturity = 4,
     MaxFee = 8,
+    Expiration = 16,
 }
 ```
 
 | name   | type                                                                              | description  |
 |--------|-----------------------------------------------------------------------------------|--------------|
-| `data` | One of [`Tip`](#tip), [`WitnessLimit`](#witnesslimit), or [`Maturity`](#maturity) | Policy data. |
+| `data` | One of [`Tip`](#tip), [`WitnessLimit`](#witnesslimit), [`Maturity`](#maturity) or [`Expiration`](#expiration) | Policy data. |
 
 ## `Tip`
 
@@ -42,6 +43,16 @@ Transaction is invalid if:
 
 - `blockheight() < maturity`
 
+## `Expiration`
+
+| name         | type     | description                              |
+|--------------|----------|------------------------------------------|
+| `expiration` | `uint32` | Block after which the transaction cannot be included. |
+
+Transaction is invalid if:
+
+- `blockheight() > expiration`
+
 ## `MaxFee`
 
 | name      | type     | description                                                                                                                                                           |