[proofs] Make LazyCDProofChain extend CDProof (#7726)

[proofs] Make LazyCDProofChain extend CDProof (#7726)

This commit makes LazyCDProofChain behave more similarly to LazyCDProof, in that it is an object that not only builds a proof to a fact based on lazy steps but also on regular steps that may already have for it.

The motivation for this commit is the upcoming handling of context-dependent proofs in incremental mode, which benefits from a more uniform behavior between CDProof, LazyCDProof and LazyCDProofChain (with the latter two both extending CDProof).