Weights for `pallet-bridge-grandpa` (#815)

* Add benchmarking skeleton

* Allow runtime to indicate concrete header type for benches

* Set up skeleton for benchmark tests

* Play around with mutating bench header

* Create a working test for benchmarking

* Add benches related to enacting authority set changes

* Add bench for checking effect of prec-commits/vote ancestries

* Use new `no_std` test utils in benchmarks

* Support pallet instances in benchmarking

* Use correct benchmarking instance macro

* Add instance to runtime benchmark helper impl

* Start using new justification creation API

* Allow mock header's number to be specified

* Set up benches with correct fork/depth parameters

* Use new pallet name during runtime bench setup

* Use correct `set_id` in tests

* Limit number of forks as workaround to get tests passing

* Use number of authorities which matches number of forks

* Make sure test post-conditions are checked properly

* Only read `CurrentAuthoritySet` from storage once

* Add combined benchmark for `submit_finality_proof`

* Add bench test

* Introduce config bounds related to justification verification

* Use config consts from pallet in benchmarking

* Return data relevant to benchmarks from helper functions

* Annotate `submit_finality_proof` with autogenerated weights

* Return actual weight after call execution

* Ignore Clippy warnings in bench template

* Update benchmark template

* Use `test-utils` to create test headers

* Clarify that helper is only for messages benches

* Add more documentation to benches

* Update TODOs

* Clarify return types in comment

* Fix pallet name post-merge

* Update NOTE to a TODO item

* Indicate that Config params are max values, not actual values

* Change Config validator count type to be `u32`

* Return decoded justification instead of fields

* Add missing trait bounds for tests

* Correctly issue weight refund

Thanks for spotting this Tomek!

* Update comment

* Add note about SESSION_LENGTH

* Fix benchmarking code