xds: implement XdsServerCredentials

[sanjaypujare]

Will remove the useXdsSecurity* methods from XdsServerBuilder in the next PR.

[ejona86]

The builder should completely stop using ProtocolNegotiator when removing the old builder methods.

[ejona86]

Actually, this exposes problems. If a user calls useTransportSecurity() it won't have any effect and won't give them an appropriate error.

[sanjaypujare]

Actually, this exposes problems. If a user calls useTransportSecurity() it won't have any effect and won't give them an appropriate error.

So instead of delegating these calls to the delegate would it be okay to create a TlsServerCredentials using the passed File arguments?

[sanjaypujare]

The builder should completely stop using ProtocolNegotiator when removing the old builder methods.

I have removed the fallbackNegotiator. Only buildServer uses/accepts a ProtocolNegotiator but that is a test-only method. Are there any other instances of the use of ProtocolNegotiator that should be removed?

[ejona86]

So instead of delegating these calls to the delegate would it be okay to create a TlsServerCredentials using the passed File arguments?

If you used the ServerCredentials factory method for NettyServerBuilder, then you wouldn't have a problem as NettyServerBuilder will fail any protocol negotiator changes when using ServerCredentials.

You need to use the proper factory method of NettyServerBuilder based on whether ServerCredentials is provided or not.

Are there any other instances of the use of ProtocolNegotiator that should be removed?

Yes, I explicitly mentioned useTransportSecurity().

[ejona86]

I rewrote the XdsServerBuilder changes. They are pretty small now, but use the old code path for the old code and new code path for new code.

Don't forget to fill out the ExperimentalApi URI.