client-sdk: allow insecure connections to localhost address

[ptrus]

This usecase came up in https://github.com/oasisprotocol/oasis-web3-gateway/pull/626 since we want to expose the node from docker via a port (since this works better cross-platform than exposing the socket via a volume).

The alternative possible solution would be to provide a flag that allows insecure connections or connections with self-signed certs (something like --insecure in curl).

BTW we might want to do the same in oasis-core: https://github.com/oasisprotocol/oasis-core/blob/280d2ef1947bc9baaeb390c3131a08b87824c9ef/go/oasis-node/cmd/common/grpc/grpc.go#L83-L89 since ideally, both oasis-node and oasis-cli would work with local-net without much hassle.

cc @matevz

TODO:

• [x] use the method from oasis-core: https://github.com/oasisprotocol/oasis-core/compare/ptrus/feature/grpc-loopback-insecure?expand=1

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[ptrus]

Please check again, I updated it a bit to be in line with the gRPC address spec defined at: https://github.com/grpc/grpc/blob/master/doc/naming.md

[ptrus]

Updated code once again, now it reuses the method from oasis-core (added in: https://github.com/oasisprotocol/oasis-core/pull/5878)

[ptrus]

Yes, but if we don't plan to release it anytime soon, then we could copy the method for now.

But we will need to release the updated oasis-sdk and oasis-cli anyway if we want a working cli with the localnet container, so we could wait for oasis-core as well if there are any plans for it in the near future.

@matevz not sure how urgently we want this for different components

• For oasis-cli to work with localnet - we need oasis-sdk and oasis-cli releases (and potentially oasis-core unless we duplicate the code here)
• For oasis-node to work with localnet - we need oasis-core release

[matevz]

Sure, we can wait for the point release of the oasis-core and keep a clean code here.

[kostko]

Oasis Core 24.3 has been released, please rebase, then we can tag the Go Client SDK.