MBM `try-runtime` support (#4251)

# MBM try-runtime support

This MR adds support to the try-runtime trait such that the
try-runtime-CLI will be able to support MBM testing
[here](https://github.com/paritytech/try-runtime-cli/pull/90). It mainly
adds two feature-gated hooks to the `SteppedMigration` hook to
facilitate testing. These hooks are named `pre_upgrade` and
`post_upgrade` and have the same signature and implications as for
single-block migrations.

## Integration

To make use of this in your Multi-Block-Migration, just implement the
two new hooks and test pre- and post-conditions in them:

```rust
#[cfg(feature = "try-runtime")]
fn pre_upgrade() -> Result<Vec<u8>, frame_support::sp_runtime::TryRuntimeError> {
	// ...
}

#[cfg(feature = "try-runtime")]
fn post_upgrade(prev: Vec<u8>) -> Result<(), frame_support::sp_runtime::TryRuntimeError> {
    // ...
}
```

You may return an error or panic in these functions to indicate failure.
This will then show up in the try-runtime-CLI and can be used in CI for
testing.

Changes:
- Adds `try-runtime` gated methods `pre_upgrade` and `post_upgrade` on
`SteppedMigration`
- Adds `try-runtime` gated methods `nth_pre_upgrade` and
`nth_post_upgrade` on `SteppedMigrations`
- Modifies `pallet_migrations` implementation to run pre_upgrade and
post_upgrade steps at the appropriate times, and panic in the event of
migration failure.

---------

Signed-off-by: Oliver Tale-Yazdi <oliver.tale-yazdi@parity.io>
Signed-off-by: georgepisaltu <george.pisaltu@parity.io>
Co-authored-by: Oliver Tale-Yazdi <oliver.tale-yazdi@parity.io>
Co-authored-by: claravanstaden <claravanstaden64@gmail.com>
Co-authored-by: ggwpez <ggwpez@users.noreply.github.com>
Co-authored-by: georgepisaltu <george.pisaltu@parity.io>