Introduce IsRemoteTrusted option to trace.StartOptions

[rakyll]

We currently provide an option for the gRPC and HTTP entry points but it is common case for users to not to trust the incoming trace ID. Rather than providing this option at integration points, provide it on the trace.StartOptions.

    // IsPublicEndpoint should be set to true for publicly accessible HTTP(S)
    // servers. If true, any trace metadata set on the incoming request will
    // be added as a linked trace instead of being added as a parent of the
    // current trace.
    IsPublicEndpoint bool

[semistrict]

We can do this without breaking the API by just deprecating the old way instead of removing it immediately. Removing the api-breaking tag.