Implement Keccak transcript

Implement the [Transcript](https://github.com/privacy-scaling-explorations/folding-schemes/tree/main/src/transcript) for Keccak.
For the use cases in which we don't need to replicate the transcript in-circuit, we can use the KeccakTranscript which will be more performant than the PoseidonTranscript.

Start by reading the linked transcript implementation and the repository's existing PoseidonTranscript to understand the expected interface and behavior. Implement the KeccakTranscript for non-circuit use cases, then verify that its transcript operations match the established transcript contract and that the intended performance-oriented use case is supported.

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