Allow usernames on approval_rules

What does this MR do and why?

Allow usernames to be used on the approval_rules API to create/update approval rules.

How to set up and validate locally

# Create a project rule
$ curl -X POST -H "Content-Type: application/json" -H "$LOCAL_TOKEN" "$LOCAL_URL/api/v4/projects/6/approval_rules" -d '{ "name": "test", "approvals_required": 3, "usernames": ["user0", "user1"]}' | jq

# Update Project Rule
$ curl -X PUT  -H "Content-Type: application/json" -H "$LOCAL_TOKEN" "$LOCAL_URL/api/v4/projects/6/approval_rules/1" -d '{ "name": "test", "approvals_required": 4, "usernames":["user0", "reported_user_15"]}' | jq

# Create a MR rule
$ curl -X POST -H "Content-Type: application/json" -H "$LOCAL_TOKEN" "$LOCAL_URL/api/v4/projects/6/merge_requests/30/approval_rules" -d '{ "name": "test", "approvals_required": 3, "usernames":["user0"]}' | jq

# Update MR Rule
$ curl -X PUT  -H "Content-Type: application/json" -H "$LOCAL_TOKEN" "$LOCAL_URL/api/v4/projects/6/merge_requests/30/approval_rules/1" -d '{ "name": "test", "approvals_required": 4, "usernames":["user0", "reported_user_15"]}' | jq

MR acceptance checklist

• I have evaluated the MR acceptance checklist for this MR.