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