RFC: Add experimental CoreML delegate

[lucyllewy]

• Update TensorFlowLiteC to TensorFlowLiteC/Core (version 2.4.0)
• Add TensorFlowLiteC/Metal to restore Metal delegate after v2.4.0 moved
  it to a separate pod.
• Add TensorflowLiteC/CoreML to add experimental CoreML delegate.

Due to being on an M1 mac, and the lack of Flutter support for the iOS simulator on M1, I'm unable to test this for sanity well yet. I'd appreciate comments and suggestions and wider testing.

[fotiDim]

Using the CoreML delegate works 👍
Using the GPUDelegate gives me a crash through:

* thread #1, queue = 'com.apple.main-thread', stop reason = EXC_BAD_ACCESS (code=1, address=0x380e1802f)
    frame #0: 0x00000001051c5930 Runner`___lldb_unnamed_symbol11964$$Runner + 1672
Runner`___lldb_unnamed_symbol11964$$Runner:
->  0x1051c5930 <+1672>: str    w12, [x9, x11]
    0x1051c5934 <+1676>: ldr    x11, [x10, #0x8]
    0x1051c5938 <+1680>: cbz    x11, 0x1051c594c          ; <+1700>
    0x1051c593c <+1684>: mov    x10, x11
Target 0: (Runner) stopped.

[lucyllewy]

@fotiDim sorry about the late reply. I pushed some code to attempt to fix your issue with GPUDelegate a while back but forgot to let you know about it - I've just rebased against the main branch to pick up the latest changes from @shaqian if you'd like to give it another pop...