Add CI job to run cargo-semver-checks (#1202)

### What
Add CI job to run cargo-semver-checks

### Why
To monitor whether the changes being made to the soroban-sdk and its
exports are a match for the version being targeted.

Close #1195

---------

Co-authored-by: Graydon Hoare <[email protected]>

.github/workflows/rust.yml

@@ -14,7 +14,7 @@ jobs:
 
   complete:
     if: always()
-    needs: [fmt, check-git-rev-deps, build-and-test, docs, readme]
+    needs: [fmt, check-git-rev-deps, semver-checks, build-and-test, docs, readme]
     runs-on: ubuntu-latest
     steps:
     - if: contains(needs.*.result, 'failure') || contains(needs.*.result, 'cancelled')
@@ -57,6 +57,17 @@ jobs:
     - name: Check if rust-analyzer encounters any errors parsing project
       run: rustup run nightly rust-analyzer analysis-stats . 2>&1 | (! grep '^\[ERROR')
 
+  semver-checks:
+    runs-on: ubuntu-latest
+    steps:
+    - uses: actions/checkout@v3
+    - run: rustup update
+    - uses: stellar/binaries@v18
+      with:
+        name: cargo-semver-checks
+        version: 0.27.0
+    - run: cargo semver-checks
+
   build-and-test:
     strategy:
       fail-fast: false