Update flatbuffers to v22.9.29 (#26)

* update flatbuffers files

How:
- Copied files in from the latest release (v22.9.29) of flatbuffers

Why:
- The files in place blocked compilation within Alpine Linux

Links:
- https://github.com/felt/tippecanoe/issues/24
- https://github.com/google/flatbuffers/releases/tag/v22.9.29

* update flatgeobuf generate files

How
- Created `feature_generated.h` & `header_generated.h` using the `src/fbs` files from the latest release (3.23.1) of flatgeobuf
    - Downloaded the `flatc` binary from the v22.9.29 release of flatbuffers
    - Copied `flatc` into the flatgeobuf repository
    - Created a new directory named `out` to collect the generated files
    - `./flatc --cpp -o out -I include src/fbs/feature.fbs`
    - `./flatc --cpp -o out -I include src/fbs/header.fbs`
    - Moved the generate files from `out` into this repository's `flatgeobuf` directory

Why
- This is a required step in the process of updating flatbuffers files

Links
- https://github.com/flatgeobuf/flatgeobuf/releases/tag/3.23.1
- https://github.com/google/flatbuffers/releases/tag/v22.9.29
- https://github.com/felt/tippecanoe/issues/24#issuecomment-1286476550

flatbuffers/vector.h

@@ -19,6 +19,7 @@
 
 #include "flatbuffers/base.h"
 #include "flatbuffers/buffer.h"
+#include "flatbuffers/stl_emulation.h"
 
 namespace flatbuffers {
 
@@ -326,6 +327,24 @@ FLATBUFFERS_CONSTEXPR_CPP11 flatbuffers::span<const uint8_t> make_bytes_span(
   return span<const uint8_t>(vec.Data(), vec.size() * sizeof(U));
 }
 
+// Convenient helper functions to get a span of any vector, regardless
+// of whether it is null or not (the field is not set).
+template<class U>
+FLATBUFFERS_CONSTEXPR_CPP11 flatbuffers::span<U> make_span(Vector<U> *ptr)
+    FLATBUFFERS_NOEXCEPT {
+  static_assert(Vector<U>::is_span_observable,
+                "wrong type U, only LE-scalar, or byte types are allowed");
+  return ptr ? make_span(*ptr) : span<U>();
+}
+
+template<class U>
+FLATBUFFERS_CONSTEXPR_CPP11 flatbuffers::span<const U> make_span(
+    const Vector<U> *ptr) FLATBUFFERS_NOEXCEPT {
+  static_assert(Vector<U>::is_span_observable,
+                "wrong type U, only LE-scalar, or byte types are allowed");
+  return ptr ? make_span(*ptr) : span<const U>();
+}
+
 // Represent a vector much like the template above, but in this case we
 // don't know what the element types are (used with reflection.h).
 class VectorOfAny {