ci: add workflow that publish rockspec

Patch adds workflow for publishing rockspec in Github Actions. Initially
similar steps were added for Travis CI in commit 'Add rock publishing'
(3728483) but later reverted in scope
of issue with discard v2.

Follows up #109
Part of #134

.github/workflows/publish.yaml

@@ -0,0 +1,39 @@
+name: Publish
+
+on:
+  push:
+    branches: [master]
+    tags: ['*']
+
+jobs:
+  publish-scm-1:
+    if: github.ref == 'refs/heads/master'
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v2
+      - uses: tarantool/rocks.tarantool.org/github-action@master
+        with:
+          auth: ${{ secrets.ROCKS_AUTH }}
+          files: http-scm-1.rockspec
+
+  publish-tag:
+    if: startsWith(github.ref, 'refs/tags/')
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v2
+      - uses: tarantool/setup-tarantool@v1
+        with:
+          tarantool-version: '2.5'
+
+      # Make a release
+      - run: echo TAG=${GITHUB_REF##*/} >> $GITHUB_ENV
+      - run: tarantoolctl rocks new_version --tag ${{ env.TAG }}
+      - run: tarantoolctl rocks install http-${{ env.TAG }}-1.rockspec
+      - run: tarantoolctl rocks pack http ${{ env.TAG }}
+
+      - uses: tarantool/rocks.tarantool.org/github-action@master
+        with:
+          auth: ${{ secrets.ROCKS_AUTH }}
+          files: |
+            http-${{ env.TAG }}-1.rockspec
+            http-${{ env.TAG }}-1.all.rock