SPIFFE/SPIRE support

Nobody has claimed this yet.

## Feature Request

Support using [SPIFFE](https://spiffe.io) for identity and use this provide mTLS credentials to services.

Other service meshes such as [Envoy](https://spiffe.io/docs/latest/microservices/envoy/), Istio and Consul support SPIFFE identities.

### What problem are you trying to solve?

Unifying identify management between multiple systems, for example Linkerd and [NATS](https://github.com/nats-io/nats-server/pull/1389) identities could be managed together and for securing cross-cluster communications.

### How should the problem be solved?

Installing Linkerd and SPIFFE to my cluster, Linkerd should get trust roots and/or certificates from SPIFFE/SPIRE directly so that I do not have to configure or provide keys to Linkerd at all.

### Any alternatives you've considered?

Working around Linkerd's inability to natively utilize SPIFFE, not using SPIFFE, or using Envoy.

### How would users interact with this feature?

Some modification to the default configuration of Linkerd so that it uses SPIFFE/SPIRE.

/ref https://github.com/linkerd/linkerd2/issues/768
/ref https://github.com/linkerd/linkerd2/issues/4667

Start by reviewing the SPIFFE/SPIRE references and linked Linkerd issues #768 and #4667, then examine how Linkerd's default configuration currently receives identity and mTLS credentials. Done means Linkerd can obtain SPIFFE/SPIRE trust roots and certificates directly, without users supplying keys, while supporting the stated cross-system identity use case.

Written by the indexing model from the issue text.