encoding: Introduce CompressorV2 API

To address #7017, the following symbols are being updated in #9036:
1. [`Compressor` interface](https://github.com/grpc/grpc-go/blob/660208049b96ff6232e8c7212905b3e357b5bf42/encoding/encoding.go#L61)
2. [`RegisterCompressor`](https://github.com/grpc/grpc-go/blob/660208049b96ff6232e8c7212905b3e357b5bf42/encoding/encoding.go#L87)
3. [`GetCompressor`](https://github.com/grpc/grpc-go/blob/660208049b96ff6232e8c7212905b3e357b5bf42/encoding/encoding.go#L95)
4. [`SetSendCompressor`](https://github.com/grpc/grpc-go/blob/660208049b96ff6232e8c7212905b3e357b5bf42/server.go#L2147)

To provide a migration path for users, the changes will be made in the following order:
- [ ] Introduce V2 symbols and deprecate V1 (symbols with no suffix): #9036
- [ ] Wait for 1 release.
- [ ] Update the V1 symbols to match the V2 signatures. Mark the V2 symbols as deprecated.
- [ ] Wait for 1 release.
- [ ] Remove the V2 symbols.

Start by reading encoding/encoding.go and server.go at the linked Compressor, RegisterCompressor, GetCompressor, and SetSendCompressor symbols, then inspect issue #9036 to understand the first migration step. Confirm which V2 symbols and deprecations remain after that work; completion follows the staged migration checklist and release waits described here.

Written by the indexing model from the issue text.