Enable Swift 6 language mode in GRPCCore (#2046)

Motivation:

v2 should use Swift 6 language mode to take full advantage of the
compilers data race checking.

Modifications:

- Enable Swift 6 language more for the code module
- Add a bunch of missing explicit 'Sendable's

Result:

- Compiles with Swift 6 lang mode

Author: glbrntt

Sources/GRPCInterceptors/HookedWriter.swift

@@ -17,7 +17,7 @@ internal import GRPCCore
 internal import Tracing
 
 @available(macOS 10.15, iOS 13, tvOS 13, watchOS 6, *)
-struct HookedWriter<Element>: RPCWriterProtocol {
+struct HookedWriter<Element: Sendable>: RPCWriterProtocol {
   private let writer: any RPCWriterProtocol<Element>
   private let beforeEachWrite: @Sendable () -> Void
   private let afterEachWrite: @Sendable () -> Void