THRIFT-5358: Add go.mod file to root directory

Client: go

This unblocks the development under go 1.16+, which starts to complain
when there's no go.mod file in any of the directories.

The current approach is certainly not the best solution ever, for
example it does not run the tests under lib/go/test/tests but copy them
into lib/go/test/gopath/src/sometest and run them there instead, but
those improvements can be done in the future in follow up PRs and this
should be a good enough first step to unblock developments.
diff --git a/build/docker/ubuntu-xenial/Dockerfile b/build/docker/ubuntu-xenial/Dockerfile
index bc66786..1184458 100644
--- a/build/docker/ubuntu-xenial/Dockerfile
+++ b/build/docker/ubuntu-xenial/Dockerfile
@@ -137,9 +137,9 @@
       libglib2.0-dev
 
 # golang
-ENV GOLANG_VERSION 1.14.14
+ENV GOLANG_VERSION 1.15.10
 ENV GOLANG_DOWNLOAD_URL https://golang.org/dl/go$GOLANG_VERSION.linux-amd64.tar.gz
-ENV GOLANG_DOWNLOAD_SHA256 6f1354c9040d65d1622b451f43c324c1e5197aa9242d00c5a117d0e2625f3e0d
+ENV GOLANG_DOWNLOAD_SHA256 4aa1267517df32f2bf1cc3d55dfc27d0c6b2c2b0989449c96dd19273ccca051d
 RUN curl -fsSL "$GOLANG_DOWNLOAD_URL" -o golang.tar.gz && \
       echo "$GOLANG_DOWNLOAD_SHA256  golang.tar.gz" | sha256sum -c - && \
       tar -C /usr/local -xzf golang.tar.gz && \