Flux.REST: Customizable pagination

[YuriyDurov]

We need to allow users to configure their pagination logic, like configuring offset/limit parameter names, allow additional pagination parameter schemas, and allow custom PageResult models mapping.

Query parameters implementation: #5

[YuriyDurov]

Some thoughts on this:

builder.Services.AddFlux(flux =>
{
    flux.AddService("MyExternalSite")
        .UsingRest("https://my-external-site.com/api/")
            .ConfigurePagination( // Global pagination config for entire Service
                (pageRequest) =>
                {
                    
                }
                (httpResponse) =>
                {
                    return PageResult;
                })
            .AddSet<Parent>()
                .WithEndpoint("parents")
                .ConfigurePagination( // pagination config for this Set
                  (pageRequest) =>
                  {
                      
                  }
                  (httpResponse) =>
                  {
                      return PageResult<Parent>;
                  })
            .AddSet<ModelDto>()
                .WithEndpoint("my-model") // https://my-external-site.com/api/my-model
                .WithPageEndpoint("parents/{parentId}/my-models")
                .ConfigurePagination( // pagination config for this Set
                  (pageRequest) =>
                  {
                      
                  }
                  (httpResponse) =>
                  {
                      return PageResult<Parent>;
                  })
});