Support TLS for csub server and clients

[dejanb]

Description of the PR

Allow CollectSub gRPC server and clients to communicate over TLS.

To start a server with TLS enabled, you need to provide a certificate and key

./bin/guaccsub --csub-tls-cert-file ./ca.crt --csub-tls-key-file ./ca.key

Clients can use --csub-tls to enable encryption. Optionally, you can spoecify --csub-tls-skip-verify to disable server certificate verification (which is needed for self-signed certs).

bin/guacone csub-client  --csub-tls --csub-tls-skip-verify ...

PR Checklist

• All commits have a Developer Certificate of Origin (DCO) -- they are generated using -s flag to git commit.
• All new changes are covered by tests
• If GraphQL schema is changed, make generate has been run
• If collectsub protobuf has been changed, make proto has been run
• All CI checks are passing (tests and formatting)
• All dependent PRs have already been merged

[lumjjb]

LGTM! Thanks @dejanb for adding this!

[pxp928]

LGTM!