[PROPOSAL] Add flag to allow merging SSO users

[strangeman]

Problem

Pritunl doesn't allow disabling of auto-creation for the SSO users (e.g. Google SSO). In that case, there may be situations when the user account will be duplicated (one account will be created via Terraform and another one by first login via SSO).

Suggestion

Add a merge_existing boolean flag to the pritunl_user resource, which will check if a user with the same set of properties (name, organization, auth_type, email) already exists and add it to the TF state if that is true.

https://registry.terraform.io/providers/cloudflare/cloudflare/4.43.0/docs/resources/record#allow_overwrite has a similar approach (but it overrides the record instead, which will break existing user profile in or case)