Automatic merge from submit-queue (batch tested with PRs 41137, 41268)

Allow the CertificateController to use any Signer implementation.

**What this PR does / why we need it**:
This will allow developers to create `CertificateController`s with arbitrary `Signer`s, instead of forcing the use of `CFSSLSigner`. It matches the behavior of allowing an arbitrary `AutoApprover` to be passed in the constructor.

**Release note**:

```release-note
NONE
```

CC @mikedanese