Add pybind11 IWYU pragmas (for clangd Include Cleaner).

Adopt the

```
// IWYU pragma: always_keep // See pybind11/docs/type_caster_iwyu.rst
```

comment used in google/pybind11clif#30073.

Currently pybind11/docs/type_caster_iwyu.rst only exists in google/pybind11clif#30073, but the intend is to upstream the changes (see PR description).

See also: https://clangd.llvm.org/design/include-cleaner

PiperOrigin-RevId: 578318836

Author: rwgk

pybind11_protobuf/enum_type_caster.h

@@ -1,3 +1,8 @@
+// IWYU pragma: always_keep // Appears to be the best we can do at the moment.
+// What we really want (but does not work with Include Cleaner @ 2023-10-27):
+// * If native_proto_caster.h is included: suggest removing enum_type_caster.h
+// * If only enum_type_caster.h is included: always_keep
+
 #ifndef PYBIND11_PROTOBUF_ENUM_TYPE_CASTER_H_
 #define PYBIND11_PROTOBUF_ENUM_TYPE_CASTER_H_
 

pybind11_protobuf/native_proto_caster.h

@@ -1,3 +1,5 @@
+// IWYU pragma: always_keep // See pybind11/docs/type_caster_iwyu.rst
+
 #ifndef PYBIND11_PROTOBUF_NATIVE_PROTO_CASTERS_H_
 #define PYBIND11_PROTOBUF_NATIVE_PROTO_CASTERS_H_
 

pybind11_protobuf/proto_cast_util.h

@@ -1,3 +1,5 @@
+// IWYU pragma: private, include "pybind11_protobuf/native_proto_caster.h"
+
 #ifndef PYBIND11_PROTOBUF_PROTO_CAST_UTIL_H_
 #define PYBIND11_PROTOBUF_PROTO_CAST_UTIL_H_
 

pybind11_protobuf/proto_caster_impl.h

@@ -1,3 +1,5 @@
+// IWYU pragma: private, include "pybind11_protobuf/native_proto_caster.h"
+
 #ifndef PYBIND11_PROTOBUF_PROTO_CASTER_IMPL_H_
 #define PYBIND11_PROTOBUF_PROTO_CASTER_IMPL_H_
 

pybind11_protobuf/wrapped_proto_caster.h

@@ -1,3 +1,5 @@
+// IWYU pragma: always_keep // See pybind11/docs/type_caster_iwyu.rst
+
 #ifndef PYBIND11_PROTOBUF_WRAPPED_PROTO_CASTER_H_
 #define PYBIND11_PROTOBUF_WRAPPED_PROTO_CASTER_H_