Add BondingRestriction config trait for staking pallet

[mnaamani]

BondingRestriction configuration trait on staking pallet to provide a means to add an external check before bonding a stash account, if other parts of the runtime might have an opinion about whether to allow it or not.

Main usecase would be if there are other pallets that lock funds and we wish to avoid any conflicting locks.

[Lezek123]

When trying to run tests (cargo test) I'm getting:

error[E0046]: not all trait items implemented, missing: `BondingRestriction`
   --> frame/offences/benchmarking/src/mock.rs:159:1
    |
159 | impl pallet_staking::Config for Test {
    | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ missing `BondingRestriction` in implementation
    |
    = help: implement the missing item: `type BondingRestriction = Type;`

[Lezek123]

I'm wondering whether it's a good idea to include controller here, since the controller account can also be changed via set_controller, in which case some controller-specific restrictions may be bypassed.

[mnaamani]

We are of course mainly concerned with the stash account, I added controller to give ability to prevent bond in-case the controller had some other lock on it which would prevent user from unbonding or doing other actions with their controller account. Obviously I didn't see the set_controller. (And coming to think of it, even if we detected it earlier, nothing would prevent user from using their controller account to stake so it would not have really prevented that anyway) I will drop controller as suggested.

[mnaamani]

Applied suggestion and fixed mocks.

cargo test --release --features runtime-benchmarks now builds and runs.
The only failing test, which also happens on polkadot-v0.9.23 without our modifications is:

test test::metadata ... FAILED

failures:

---- test::metadata stdout ----
thread 'test::metadata' panicked at 'assertion failed: `(left == right)`

Diff < left / right > :
 Some(
     Variant(
         TypeDefVariant {
             variants: [
                 Variant {
                     name: "set_dummy",
                     fields: [
                         Field {
                             name: Some(
                                 "new_value",
                             ),
                             ty: UntrackedSymbol {
                                 id: 38,
                                 marker: PhantomData,
                             },
                             type_name: Some(
<                                "T::Balance",
>                                "/*«*/ T::Balance /*»*/",
                             ),
                             docs: [],
                         },
                     ],
                     index: 0,
                     docs: [],
                 },
             ],
         },
     ),
 )

', frame/support/test/tests/pallet_compatibility.rs:299:13
note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace


failures:
    test::metadata

test result: FAILED. 2 passed; 1 failed; 0 ignored; 0 measured; 0 filtered out; finished in 0.00s

error: test failed, to rerun pass '-p frame-support-test --test pallet_compatibility'

[Lezek123]

LGTM