Support ECDSA P-256 certificates

**Describe the problem/challenge you have**

I need to bootstrap Linkerd with an ECDSA P-256 certificate.

**Describe the solution you'd like**

I would like to use the secretgen controller to create a ECDSA P-256 certificate for use by graduated CNCF project Linkerd. Linkerd does not support the RSA certs created by the secretgen controller.

**Anything else you would like to add:**