gRPC over TLS support in clangd

@ArcsinX, as discussed in https://discourse.llvm.org/t/rfc-clangd-tls-and-kerberos-auth-for-remote-index-client/91413/7 to use `clangd-index-server` in corporate scenarios traffic between `clangd` and `clangd-index-server` have to be encrypted and the peer verified.

Supposed interface changes.

On client side: `--remote-index-cacert=/etc/ssl/trusted-ca.cer`
On server side: `--tls-cert=/etc/ssl/server.pem`

Start by reading the linked RFC and tracing the remote index client in clangd and the corresponding clangd-index-server entry points. The proposed client and server flags define the visible scope; done means traffic is encrypted and the peer is verified for both sides.

Written by the indexing model from the issue text.