Move SvixHttpClient to internal package

[svix-mman]

This is done in preparation for the new internal API client

In Go, the internal package has special semantics. https://pkg.go.dev/cmd/go#hdr-Internal_Directories
So we can define public method for our SDK to consume, but external packages wont be able to import the internal package
For example this is the error I get when trying to import the internal package in the TF provider

package github.com/svix/terraform-provider-svix
        main.go:10:3: use of internal package github.com/svix/svix-webhooks/go/internal not allowed

This is technically a breaking change, because the svix.SvixHttpClient struct will no longer be available
However, this is not an issue. Since there is no use in defining the svix.SvixHttpClient by hand, because the only use case it has is to be part of the resource structs

type Application struct {
	client *SvixHttpClient
}

But a user of our SDK would not be define the Application struct, because client is not an exported field.

[svix-jplatte]

Makes sense. Why were these public in the first place? Should they not have been lowerCamelCase private symbols?

[svix-mman]

Why were these public in the first place?

Not sure 🤷

Should they not have been lowerCamelCase private symbols?

Yup