Add registry certificate verification support

[5p2O5pe25ouT]

I want to help add support for carrying certificates when Syft scans the registry. Supported configurations are:

...MTLS for a specific registry:

# .syft.yaml
registry:
  ca-cert: "./ca-certs/myreg.crt"     # trust a specific CA cert

  auth:
   # note: no user/pass was provided, so the keychain will be used for basic auth
    - authority: "myregistry.com"
      tls-cert: "./certs/myreg-client.crt"
      tls-key: "./certs/myreg-client.key"

... MTLS for multiple registries:

# .syft.yaml
registry:
  ca-cert: "./ca-certs"     # trust all in all *.crt files in a dir as root CAs

  auth:

    - authority: "myregistry.com"
      tls-cert: "./certs/myreg-client.crt"
      tls-key: "./certs/myreg-client.key"

    - authority: "otherreg.com"
      tls-cert: "./certs/other-client.crt"
      tls-key: "./certs/other-client.key"

... configure via environment variables:

export SYFT_REGISTRY_CA_CERT="./ca-certs/"
export SYFT_REGISTRY_AUTH_TLS_CERT="./certs/myreg-client.crt"
export SYFT_REGISTRY_AUTH_TLS_KEY="./certs/other-client.key"

which will be the same as:

registry:
  ca-cert: "./ca-certs"   

  auth:

    - authority: ""
      tls-cert: "./certs/myreg-client.crt"
      tls-key: "./certs/myreg-client.key"

...which will offer the client cert during TLS negotiation to any registry that will accept it.

Pulls in features from anchore/stereoscope#169 and anchore/stereoscope#195