BitOps Plugin support for private repos

Need to implement a pattern in `plugin.config.yml` to support private plugin repos. For example:

```
plugins:
- name: pluginA
repo: https://github.com/myorg/pluginA.git
auth:
# type http or ssh detected by repo prefix
user:
type: string
value: myusername
user:
type: string
value: mytoken
- name: pluginB
repo: https://github.com/myorg/pluginB.git
auth:
user:
type: env
value: $GIT_USER #defines which env var to get this value from
user:
type: env
value: $GIT_PASSWORD #defines which env var to get this value from
- name: pluginC
repo: git@github.com:myorg/pluginC.git
auth:
sshkey:
type: string
value: |
-----BEGIN RSA PRIVATE KEY-----
...
-----END RSA PRIVATE KEY-----
- name: pluginD
repo: git@github.com:myorg/pluginD.git
sshkey:
type: env
value: $GIT_SSHKEY #defines which env var to get this value from
- name: pluginE
repo: git@github.com:myorg/pluginD.git
sshkey:
type: path
value: /path/to/mounted/sshkey
```