Introduce cargo mutants cron job (payjoin#573)

Relates to payjoin#539

This is our first step at cargo mutants in our ci!

At the moment all cargo mutant exploration I have been doing locally but
with over 250 missed mutants remaining and possibly growing as new code
is introduced I think it is time to implement a weekly cron job to track
these mutants.

This heavily copies the rust-bitcoin implementation of their mutants
workflow with tweaks for our codebase and mutants coverage progress.

currently set to happen weekly Sun 00:00

Author: spacebear21

.cargo/mutants.toml

@@ -0,0 +1,15 @@
+additional_cargo_args = ["--all-features"]
+examine_globs = ["payjoin/src/uri/*.rs"]
+exclude_globs = []
+exclude_re = [
+	"impl Debug",
+	"impl Display",
+	"deserialize",
+	"Iterator",
+	".*Error",
+
+	# ---------------------Crate-specific exculsions---------------------
+	# Receive
+	# src/receive/v1/mod.rs
+        "interleave_shuffle", # Replacing index += 1 with index *= 1 in a loop causes a timeout due to an infinite loop
+]

.github/workflows/cron-weekly-mutants.yml

@@ -0,0 +1,39 @@
+name: Weekly cargo-mutants
+on: 
+  schedule: 
+    - cron: "0 0 * * 0" # runs weekly on Sunday at 00:00
+  workflow_dispatch: # allows manual triggering
+jobs:
+  cargo-mutants:
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v4
+      - uses: taiki-e/install-action@v2
+        with:
+          tool: cargo-mutants
+      - run: cargo mutants --in-place --no-shuffle
+      - uses: actions/upload-artifact@v4
+        if: always()
+        with:
+          name: mutants.out
+          path: mutants.out
+      - name: Check for new mutants
+        if: always()
+        run: |
+          if [ -s mutants.out/missed.txt ]; then
+            echo "New missed mutants found"
+            gh issue create \
+            --title "New Mutants Found" \
+            --body "$(cat <<EOF
+          Displaying up to the first 15 missed mutants:
+          $(head -n 15 mutants.out/missed.txt)
+          For the complete list, please check the [mutants.out artifact](https://github.com/${{ github.repository }}/actions/runs/${{ github.run_id }}).
+          EOF
+          )"
+            echo "create_issue=true" >> $GITHUB_ENV
+          else
+            echo "No new mutants found"
+            echo "create_issue=false" >> $GITHUB_ENV
+          fi
+    env:
+      GITHUB_TOKEN: ${{ secrets.GITHUB_TOKEN }}

.gitignore

@@ -4,3 +4,4 @@ target
 *payjoin.sled
 Cargo.lock
 .vscode
+mutants.out