Add benchmarks to pipeline

[alvicsam]

PR adds manual benchmarks job. It's divided to 2 jobs: benchmarks-build which builds binary on a faster ci runners and passes to benchmarks job which execute benchmarks.
The benchmarks job runs script benchmarks-ci.sh for statemine, statemint and westmint and creates PR on github with updated weights (example: #1138)

Part of https://github.com/paritytech/ci_cd/issues/260
Closes #631

[alvicsam]

There is an additional condition that should be implemented (after discussing it with @NachoPal in element):

Weights should be pushed to the same branch where the binary for benchmarks was compiled.

[NachoPal]

Thanks @alvicsam . I realised that we'll need an input to select the branch we want to use for building the binary, running the benchmarks and opening a PR with the new weights.

The flow would be as follows:

1. Select branch release-parachains-vX.Y.Z -> Click Run benchmarks button
2. The binary is built, benchmarks are run.
3. When the new weights are ready, a PR is open against release-parachains-vX.Y.Z
4. We review the PR and merge it to release-parachains-vX.Y.Z

[alvicsam]

@NachoPal I changed benchmarks jobs. Now they will appear only on pipelines that are related to release-parachains-v* branches and behave as you described (run benchmarks, create PR against release-parachains-v*)

[NachoPal]

Thanks!

[NachoPal]

What Benchmarking machine is used to run the benchmarks?

[alvicsam]

bm4, bm5, bm6