Create GH Actions workflow for build&test of Go client

As described in RFC-18, there is a need for a refactorization of Keep
and tBTC release processes in order to reduce human involvment and make
the processes faster and less error prone. A part of this task is
migration from CircleCI jobs to GitHub Actions. This commit creates a
GitHub Action workfow for building and testing Go client.

.github/workflows/client.yml

@@ -0,0 +1,30 @@
+name: Go
+
+#TODO: extend the conditions once workflow gets tested together with other workflows 
+on: [push, pull_request]
+
+jobs:
+  build-and-test:
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v1
+      - uses: satackey/[email protected]
+        continue-on-error: true # ignore the failure of a step and avoid terminating the job
+      - name: Create test results directory
+        run: |
+          mkdir -p /tmp/test-results/keep-core-go
+      - name: Run Docker build
+        run: |
+          docker build --target gobuild -t go-build-env .
+          docker build --tag keep-client . 
+      - name: Run Go tests
+        run: |
+          docker run --volume /tmp/test-results/keep-core-go:/mnt/test-results --workdir /go/src/github.com/keep-network/keep-core go-build-env gotestsum --junitfile /mnt/test-results/unit-tests.xml
+          docker run --volume /tmp/test-results/keep-core-go:/mnt/test-results --workdir /go/src/github.com/keep-network/keep-core go-build-env cat /mnt/test-results/unit-tests.xml > ./unit-tests.xml
+      - name: Publish Unit Test results
+        uses: EnricoMi/[email protected]
+        if: always() # guarantees that this action always runs, even if earlier steps fail
+        with:
+          github_token: ${{ secrets.GITHUB_TOKEN }}
+          files:  unit-tests.xml
+          check_name: Go Test Results # name under which test results will be presented in GitHub (optional)