Feature: ACL Policy Management allow overwriting/looking up a policy by name

#### Feature Description

The Consul Agent CLI supports the no-merge feature when executing an UPDATE command on an existing ACL policy. (https://www.consul.io/docs/commands/acl/policy/update.html#no-merge)
I'd like to have that feature also available on the API of Consul.

#### Use Case(s)

We're provisioning policies through the API and wanted to update policies in place but overwriting the old rules.
Currently, we have to:

1. check if we have a policy existing with our naming convention
2. delete it
3. recreate it with the new rules

We'd just like to avoid having to delete an existing policy when we only want to overwrite rules.

Start by comparing the Consul Agent CLI ACL policy UPDATE command and its no-merge behavior with the existing Consul API policy update entry point. Define how API callers can overwrite rules and look up a policy by name, then verify that updating an existing policy avoids the delete-and-recreate workflow described in the issue.

Written by the indexing model from the issue text.