feat(csharp/Reflection): change MessageParser.CreateTemplate to be public

[ahmednfwela]

this is a subset of the changes in #7722 where MessageParser.CreateTemplate is made public, this removes the need to use reflection to create protobuf messages given a descriptor/parser.

future work: maybe for the next major release we can change the generated code to use MessageParser.CreateTemplate, fully solving #6401.

[JasonLunn]

Thank you for your contribution, @ahmednfwela. Although this PR is small, the impact of expanding the API surface could be significant and we need additional time to review it to make sure it aligns with our long term goals across the languages that Protobuf supports.

[esrauchg]

Unfortunately our C# expertise is somewhat bandwidth limited right now, but as a lower-c#-context protobuf team member it seems a bit odd to me that this method should be a public method on Parser, since this is not really a parse related operation.

A similar shape of method is commonly an instance method on Message type, eg in Java var builder = msg.newBuilderForType() and in C++ the Message* m = msg.New() method (where there is a common pattern to pass around a SomeMsg.defaultInstance() as a prototype and then newBuilderForType()/New() on that in code that wants to create more instances of the same type).

Just to understand the possible design scope better, would a method on IMessage that is parallel to those precedents also solve for the need that you have?

[ahmednfwela]

@esrauchg
The whole point of this PR is to enable the creation of an arbitrary message given its parser (by exposing the factory method of the parser).

This opens the door for something like #7722 to unify the creation interface, so that the consumer can modify the factory logic to their needs (e.g. introducing reusable message pools).

I would personally not recommend moving the method on IMessage, because it forces us to have an instance of IMessage to be able to create an empty message .

But having it on the parser is similar to having an IMessageFactory which is a much better design