This commit fixes#3369, refs #3374
It adds support for kuberenetes_users section in roles,
allowing Teleport proxy to impersonate user identities.
It also extends variable interpolation syntax by adding
suffix and prefix to variables and function `email.local`:
Example:
```yaml
kind: role
version: v3
metadata:
name: admin
spec:
allow:
# extract email local part from the email claim
logins: ['{{email.local(external.email)}}']
# impersonate a kubernetes user with IAM prefix
kubernetes_users: ['IAM#{{external.email}}']
# the deny section uses the identical format as the 'allow' section.
# the deny rules always override allow rules.
deny: {}
```
Some notes on email.local behavior:
* This is the only function supported in the template variables for now
* In case if the email.local will encounter invalid email address,
it will interpolate to empty value, will be removed from resulting
output.
Changes in impersonation behavior:
* By default, if no kubernetes_users is set, which is a majority of cases,
user will impersonate themselves, which is the backwards-compatible behavior.
* As long as at least one `kubernetes_users` is set, the forwarder will start
limiting the list of users allowed by the client to impersonate.
* If the users' role set does not include actual user name, it will be rejected,
otherwise there will be no way to exclude the user from the list).
* If the `kuberentes_users` role set includes only one user
(quite frequently that's the real intent), teleport will default to it,
otherwise it will refuse to select.
This will enable the use case when `kubernetes_users` has just one field to
link the user identity with the IAM role, for example `IAM#{{external.email}}`
* Previous versions of the forwarding proxy were denying all external
impersonation headers, this commit allows 'Impesrsonate-User' and
'Impersonate-Group' header values that are allowed by role set.
* Previous versions of the forwarding proxy ignored 'Deny' section of the roles
when applied to impersonation, this commit fixes that - roles with deny
kubernetes_users and kubernetes_groups section will not allow
impersonation of those users and groups.
Alexander Klizhentas