[Swift] Migrates struct write APIS to write directly to the buffer (#6093)

* Migrates struct write APIS to in place APIS

* Fixes indentation in grpc swift

tests/FlatBuffers.Benchmarks.swift/Sources/FlatBuffers.Benchmarks.swift/main.swift

@@ -60,6 +60,34 @@ func createDocument(Benchmarks: [Benchmark]) -> String {
     }
 }
 
+@inlinable func benchmarkThreeMillionStructs() {
+    let structCount = 3_000_000
+    
+    let rawSize = ((16 * 5) * structCount) / 1024
+    
+    var fb = FlatBufferBuilder(initialSize: Int32(rawSize * 1600))
+    
+    var offsets: [Offset<UOffset>] = []
+    for _ in 0..<structCount {
+        fb.startVectorOfStructs(count: 5, size: 16, alignment: 8)
+        for _ in 0..<5 {
+            fb.createStructOf(size: 16, alignment: 8)
+            fb.reverseAdd(v: 2.4, postion: 0)
+            fb.reverseAdd(v: 2.4, postion: 8)
+            fb.endStruct()
+        }
+        let vector = fb.endVectorOfStructs(count: 5)
+        let start = fb.startTable(with: 1)
+        fb.add(offset: vector, at: 4)
+        offsets.append(Offset<UOffset>(offset: fb.endTable(at: start)))
+    }
+    let vector = fb.createVector(ofOffsets: offsets)
+    let start = fb.startTable(with: 1)
+    fb.add(offset: vector, at: 4)
+    let root = Offset<UOffset>(offset: fb.endTable(at: start))
+    fb.finish(offset: root)
+}
+
 func benchmark(numberOfRuns runs: Int) {
     var benchmarks: [Benchmark] = []
     let str = (0...99).map { _ -> String in return "x" }.joined()
@@ -68,6 +96,7 @@ func benchmark(numberOfRuns runs: Int) {
     let hundredStr = run(name: "100 str", runs: runs) {
         create100Strings(str: str)
     }
+    benchmarks.append(run(name: "3M strc", runs: 1, action: benchmarkThreeMillionStructs))
     benchmarks.append(hundredStr)
     print(createDocument(Benchmarks: benchmarks))
 }