Create `RangedIndex` interface used in C# codegen similar to Rust

Currently, the module-side generated data structures for indexes don't implement a single interface, despite all having the same interface. The generated code for these looks like

public sealed class nameIndex() : SpacetimeDB.Internal.IndexBase<global::SpacetimeDB.Modules.ModuleTestCs.TestE>("test_e_name_idx_btree") {
    public IEnumerable<global::SpacetimeDB.Modules.ModuleTestCs.TestE> Filter(string name) =>
        DoFilter(new SpacetimeDB.Internal.BTreeIndexBounds<string, SpacetimeDB.BSATN.String>(name));

    public ulong Delete(string name) =>
        DoDelete(new SpacetimeDB.Internal.BTreeIndexBounds<string, SpacetimeDB.BSATN.String>(name));

    public IEnumerable<global::SpacetimeDB.Modules.ModuleTestCs.TestE> Filter(Bound<string> name) =>
        DoFilter(new SpacetimeDB.Internal.BTreeIndexBounds<string, SpacetimeDB.BSATN.String>(name));

    public ulong Delete(Bound<string> name) =>
        DoDelete(new SpacetimeDB.Internal.BTreeIndexBounds<string, SpacetimeDB.BSATN.String>(name));

}

Either IndexBase should grow more methods, an interface should be added, or this sort of index should be converted to a single data structure.
This is a wart that could be fixed backwards-compatibly since it is only visible inside modules.

Start by locating the C# code-generation entry point for module-side index structures and compare it with the similar Rust interface referenced in the issue. Determine whether RangedIndex or another backwards-compatible approach best unifies the generated indexes. Done means the generated C# indexes share the intended interface without breaking existing module code; no file or test is named in the issue.