[AHM-Kusama] EPMB: invulnerables and offchain storage

[kianenigma]

This PR bring two new features to the election-provider-multi-block pallets, based on audit reviews and our empirical experience in westend:

1. While I have not found the actual root cause, the offchain-worker miner in westend collators is sometimes unreliable. In this PR, I have added a configuration that would disable the call cache for this offchain worker. I have run this new code in my personal WAH collator, and it is indeed more reliable. This just gives us more lever to easily disable the cache. Investigation should still continue as to what is the root cause.
2. the signed submission system now supports a governance set of invulnerables. See the code comments for what this implies.

Marking this as silent for now as these crates are unreleased for faster pace, but can add prdoc if needed.

This will be quite useful to lower the chance of backports after #8764 deadline, but is not mandatory.

It partially closes #8879

[sigurpol]

Investigation should still continue as to what is the root cause.

Have you already created an issue for that so that we can track it and we (to be read as : I...) don't forget ?

[paritytech-workflow-stopper]

All GitHub workflows were cancelled due to failure one of the required jobs.
Failed workflow url: https://github.com/paritytech/polkadot-sdk/actions/runs/15706725409
Failed job name: build-rustdoc

[kianenigma]

Investigation should still continue as to what is the root cause.

Have you already created an issue for that so that we can track it and we (to be read as : I...) don't forget ?

great point! created #8880

[sigurpol]

A general comment: I’m not sure if we are too late for this and I am fine with reviewing the PR as it is, but I would have appreciated two separate PRs, as they address two completely unrelated topics. E.g. I am done with the review of OCW part and beside a comment on a test to extend, I am pretty sure it can be merged immediately. I haven't started on the invulnerable part but that might require further discussion etc.
It's also true that I can review the first two commits independently and achieve more or less the same 😄

[kianenigma]

A general comment: I’m not sure if we are too late for this and I am fine with reviewing the PR as it is, but I would have appreciated two separate PRs, as they address two completely unrelated topics. E.g. I am done with the review of OCW part and beside a comment on a test to extend, I am pretty sure it can be merged immediately. I haven't started on the invulnerable part but that might require further discussion etc. It's also true that I can review the first two commits independently and achieve more or less the same 😄

Yeah let's see -- if your review of the second commit turns out complicated, I can split the PRs.

[sigurpol]

LGTM, some minor comments mostly about adding tests.
Not a blocker on my side

[sigurpol]

A general comment: I’m not sure if we are too late for this and I am fine with reviewing the PR as it is, but I would have appreciated two separate PRs, as they address two completely unrelated topics. E.g. I am done with the review of OCW part and beside a comment on a test to extend, I am pretty sure it can be merged immediately. I haven't started on the invulnerable part but that might require further discussion etc. It's also true that I can review the first two commits independently and achieve more or less the same 😄

Yeah let's see -- if your review of the second commit turns out complicated, I can split the PRs.

It turned out to be unnecessary in the end; it was quite small. I complained for nothing. 😆

[kianenigma]

All new changes make sense to me, big thanks @sigurpol!

@seadanda PTAL and merge once you are good with this :)