perf: optimize GenerateRandomString()

The previous implementation used an std::stringstream to concatenate
the generated random string.

The new one uses a simple preallocated std::string, as the size of the
output is already known - it is the length parameter.

It also uses std::generate_n() instead of an explicit loop, making the
code more concise and potentially faster, as no calls to
std::string::operator+= are needed.

Calling GenerateRandomString(1'000'000) with the std::stringstream-based
implementation allocated 16 times, for a total of 3'173'516 bytes.
The new one cuts this down to 4 allocs, for a total of 1'076'864 bytes.

src/util/helpers/helpers.cpp

@@ -417,18 +417,20 @@ std::string GenerateRandomString(size_t length)
 	return GenerateRandomString(length, kCharacters);
 }
 
-std::string GenerateRandomString(size_t length, std::string_view characters)
+std::string GenerateRandomString(const size_t length, const std::string_view characters)
 {
 	assert(!characters.empty());
-	std::stringstream result;
+	std::string result;
+	result.resize(length);
 
 	std::random_device rd;
 	std::mt19937 gen(rd());
 	std::uniform_int_distribution<decltype(characters.size())> index_dist(0, characters.size() - 1);
-	for (uint32_t i = 0; i < length; ++i)
+	std::generate_n(
-	{
+		result.begin(),
-		result << characters[index_dist(gen)];
+		length,
-	}
+		[&] { return characters[index_dist(gen)]; }
+	);
 
-	return result.str();
+	return result;
 }