Allow UnifiedMessages to be transported over webapi

[xPaw]

The point of unified messages is that they can work both over CM and webapi, and Steam does this have implementing underlying "service transport"

We have WebAPI method that does Task<T> CallProtobufAsync<T> where you have to manually provide the request and response types.

We can make this nicer (after #1433), that the calling could would be similar, but the request would be sent over http instead of cm.

SK uses it here for example:

SteamKit/SteamKit2/SteamKit2/Steam/WebAPI/ContentServerDirectoryService.cs

Line 77 in 0931a59

var response = await directory.CallProtobufAsync<CContentServerDirectory_GetServersForSteamPipe_Response, CContentServerDirectory_GetServersForSteamPipe_Request>(

[xPaw]

There's a problem with this: we have to choose GET or POST for methods. Valve's js does this by having "bConstMethod" on each SendMsg call, but I don't know where to get this info from client protobuf dumps (it's not in the method options).