[summary] Track CI job runtime (#2936)

## Motivation

It would be nice to see CI runtime regressions or improvements in the PRs.

## Proposal

Create Rust CLI tool that checks runtimes, compares against base ref, and comments on the PR with a summary of the changes.
This works, but as you guys can see it's pretty ugly for now, we're just posting the JSON 😅
We're also commenting for every successful workflow run in CI.

The PR is already pretty big so I will do the following follow ups:
- [x] Markdown format this in a pretty way (done in #2946)
- [x] Add the performance summary markdown as a [job summary](https://github.blog/news-insights/product-news/supercharging-github-actions-with-job-summaries/) of the job, so people have access to historical versions of this (done in #2948)
- [x] Find if there's already a performance summary comment, and edit it instead of always posting a new one (done in #2953)
- [ ] Might make sense to only post if the change exceeds some percentage threshold for now. Open to suggestions

Other follow ups are tracked in the issue for this: #2834

## Test Plan

Triggered a modified version of this manually with workflow dispatch, it works, but we can unfortunately only know for sure this works by merging :/ 

## Release Plan

- Nothing to do / These changes follow the usual release cycle.

.github/workflows/performance_summary.yml

@@ -0,0 +1,38 @@
+name: Performance Summary
+
+on:
+  workflow_run:
+    workflows:
+      - Rust
+    types:
+      - completed
+  workflow_dispatch:
+
+# This allows a subsequently queued workflow run to interrupt previous runs on pull-requests
+concurrency:
+  group: '${{ github.workflow }} @ ${{ github.head_ref || github.event.workflow_run.head_commit.id || github.event.workflow_run.id || github.run_id }}'
+  cancel-in-progress: true
+
+permissions:
+  contents: read
+  pull-requests: write
+
+jobs:
+  performance-summary:
+    runs-on: ubuntu-latest
+    if: ${{ github.event.workflow_run.conclusion == 'success' }}
+    steps:
+      - uses: actions/checkout@v3
+      - uses: actions-rust-lang/setup-rust-toolchain@v1
+      - name: Post Performance Summary comment on PR
+        if: ${{ github.event.workflow_run.event == 'pull_request' }}
+        env:
+          GITHUB_TOKEN: ${{ secrets.GITHUB_TOKEN }}
+          GITHUB_REPOSITORY: ${{ github.repository }}
+          GITHUB_PR_NUMBER: ${{ github.event.workflow_run.pull_requests[0].number }}
+          GITHUB_BASE_BRANCH: ${{ github.event.workflow_run.pull_requests[0].base.ref}}
+          GITHUB_PR_BRANCH: ${{ github.event.workflow_run.pull_requests[0].head.ref }}
+          GITHUB_PR_COMMIT_HASH: ${{ github.event.workflow_run.pull_requests[0].head.sha }}
+        # The list of workflows that trigger this need to be hardcoded above.
+        # Make sure you have the same comma separated list of workflows here.
+        run: cargo run -p linera-summary -- --workflows "Rust" ci