Move certs helper code from /internal/examples to /internal or /internal/testhelper?

[srikanthccv]

In an effort to improve the test coverage for serverimpl, I started to add some tests with TLS enabled. I noticed we already have some useful code for generating and reading certs in internal/examples/..., and it seems like a good idea to use for tests. I was thinking of moving them to /internal or maybe internal/testhelper and use in both serverimpl tests and examples.

@tigrannajaryan do you see any issues with this? If not, I can send a PR for this along with the new serverimpl TLS tests.

[tigrannajaryan]

Sounds good to me.