ComposedPhysicalExtensionCodec should not rely on positional encoding

[jayshrivastava]

Is your feature request related to a problem or challenge?

ComposedPhysicalExtensionCodec should identify codecs by stable ID instead of list position

ComposedPhysicalExtensionCodec records the position of each codec in the DataEncoderTuple:

struct DataEncoderTuple {
    pub encoder_position: u32,
    pub blob: Vec<u8>,
}

The problem is that if someone adds, removes, or reorders a codec, you get really confusing error messages. For example:
1. A writer with [CodecA, CodecB] encodes protobuf `A` and specifies `encoder_position: 0`
2. A reader with [CodecB, CodecA] decodes the proto using `encoder_position: 0` and errors with "CodecB cannot decode message"

It's hard to see that this was caused a breaking protocol change. Since both codecs are present, it doesn't seem like a breaking change, but it is.

Describe the solution you'd like

Maybe we can introduce an id to look up encoders:

struct DataEncoderTuple {
      // Retained for decoding payloads written by older versions.
      pub encoder_position: u32,
      pub blob: Vec<u8>,
      pub codec_id: Option<String>,
  }

  Decoding would:

  1. Use codec_id when present.
  2. Return an explicit error containing the unknown identifier when it is not registered.
  3. Fall back to encoder_position for legacy payloads.

Describe alternatives you've considered

No response

Additional context

No response

[imtherealnaska]

take

[jayshrivastava]

@imtherealnaska Because this is a significant change, it may be good to get a committer's opinion on if it's a good idea first :) cc @adriangb

[imtherealnaska]

yeah true 👍🏼

[gabotechs]

Was just coincidentally drafting something about this here: #24826