Prototype of customizable recursion and size limits for message parsing

[jskeet]

cc @JamesNK @EronWright

Obviously this has no tests, but it's an alternative to #17881.

I don't know how the gRPC generator would be configured to change the parser settings, but the result could be a change from:

[global::System.CodeDom.Compiler.GeneratedCode("grpc_csharp_plugin", null)]
static readonly grpc::Marshaller<global::Google.Cloud.Functions.V2.GetFunctionRequest> 
    __Marshaller_google_cloud_functions_v2_GetFunctionRequest =
    grpc::Marshallers.Create(__Helper_SerializeMessage, context =>
        __Helper_DeserializeMessage(context, global::Google.Cloud.Functions.V2.GetFunctionRequest.Parser));

to

[global::System.CodeDom.Compiler.GeneratedCode("grpc_csharp_plugin", null)]
static readonly grpc::Marshaller<global::Google.Cloud.Functions.V2.GetFunctionRequest> 
    __Marshaller_google_cloud_functions_v2_GetFunctionRequest =
    grpc::Marshallers.Create(__Helper_SerializeMessage, context =>
        __Helper_DeserializeMessage(context, global::Google.Cloud.Functions.V2.GetFunctionRequest.Parser
            .WithRecursionLimit(500)));

Causes for concern (beyond the current lack of tests):

• Additional complexity in MessageExtensions and CodedInputStream
• Potential performance regression due to dereferencing Settings frequently
• Having an "immutable" Settings type referring to a mutable ExtensionRegistry is a code smell, but that's an existing issue.

[JasonLunn]

Any update on this, @jskeet ?

[jskeet]

@JasonLunn: I haven't had any feedback, and don't have the time to chase at the moment I'm afraid.

[mkruskal-google]

This looks reasonable to me, although tests would be nice

[jskeet]

@mkruskal-google: I'll definitely add tests before we merge :) I'd like feedback on the API surface from potential consumers such as @JamesNK before writing those tests though.

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