[C++][FS][Azure] Add support for reading user defined metadata

[kou]

Describe the enhancement requested

We've implemented ReadMetadata() by #38330.
It uses properties not user defined metadata. But our generic test has a test that writes/reads user defined metadata:

arrow/cpp/src/arrow/filesystem/test_util.cc

Lines 911 to 927 in bb9f229

	// Storing metadata along file
	auto metadata = KeyValueMetadata::Make({"Content-Type", "Content-Language"},
	{"x-arrow/filesystem-test", "fr_FR"});
	ASSERT_OK_AND_ASSIGN(stream, fs->OpenOutputStream("jkl", metadata));
	ASSERT_OK(stream->Write("data"));
	ASSERT_OK(stream->Close());
	ASSERT_OK_AND_ASSIGN(auto input, fs->OpenInputStream("jkl"));
	ASSERT_OK_AND_ASSIGN(auto got_metadata, input->ReadMetadata());
	if (have_file_metadata()) {
	ASSERT_NE(got_metadata, nullptr);
	ASSERT_GE(got_metadata->size(), 2);
	ASSERT_OK_AND_EQ("x-arrow/filesystem-test", got_metadata->Get("Content-Type"));
	} else {
	if (got_metadata) {
	ASSERT_EQ(got_metadata->size(), 0);
	}
	}

Component(s)

C++

[Tom-Newton]

I think this duplicates #40026

[kou]

Oh, sorry.