feat: Generate ServiceNameClient interface akin to gRPC's plugin

[meling]

The gRPC plugin generates a ServiceNameClient interface like this one:

// MultiPaxosClient is the client API for MultiPaxos service.
//
// For semantics around ctx use and closing/ending streaming RPCs, please refer to https://pkg.go.dev/google.golang.org/grpc/?tab=doc#ClientConn.NewStream.
type MultiPaxosClient interface {
	Prepare(ctx context.Context, in *PrepareMsg, opts ...grpc.CallOption) (*PromiseMsg, error)
	Accept(ctx context.Context, in *AcceptMsg, opts ...grpc.CallOption) (*LearnMsg, error)
	Commit(ctx context.Context, in *LearnMsg, opts ...grpc.CallOption) (*Empty, error)
	ClientHandle(ctx context.Context, in *Value, opts ...grpc.CallOption) (*Response, error)
}

Such an interface can be useful for testing.

Since the Gorums plugin replaces the gRPC plugin because it generates overlapping types, we should provide the relevant interface as part of code generation in the _gorums.pb.go file. This should be straightforward since we already generate methods on the Configuration type, such as this one:

var quorumCallSignature = `func (c *Configuration) {{$method}}(` +
	`ctx {{$context}}, in *{{$in}}` +
	`{{perNodeFnType .GenFile .Method ", f"}})` +
	`(resp *{{$customOut}}, err error) {
`

We already generate the corresponding server-side interface here:

var serverInterface = `
{{$genFile := .GenFile}}
{{range .Services -}}
{{$service := .GoName}}
// {{$service}} is the server-side API for the {{$service}} Service
type {{$service}} interface {
	{{- range .Methods}}
	{{- if isOneway .}}
	{{.GoName}}(ctx {{$context}}, request *{{in $genFile .}})
	{{- else if correctableStream .}}
	{{.GoName}}(ctx {{$context}}, request *{{in $genFile .}}, send func(response *{{out $genFile .}}) error) error
	{{- else}}
	{{.GoName}}(ctx {{$context}}, request *{{in $genFile .}}) (response *{{out $genFile .}}, err error)
	{{- end}}
	{{- end}}
}
{{- end}}
`

[meling]

We should also rename the server-side interface name to ServiceNameServer to align with the gRPC name:

// MultiPaxosServer is the server API for MultiPaxos service.
// All implementations must embed UnimplementedMultiPaxosServer
// for forward compatibility.
type MultiPaxosServer interface {
	Prepare(context.Context, *PrepareMsg) (*PromiseMsg, error)
	Accept(context.Context, *AcceptMsg) (*LearnMsg, error)
	Commit(context.Context, *LearnMsg) (*Empty, error)
	ClientHandle(context.Context, *Value) (*Response, error)
	mustEmbedUnimplementedMultiPaxosServer()
}