chore: add errcheck for parsing cli flags in plugins

[thevilledev]

Add a missing error check to plugin examples when parsing CLI flags. At the moment a parse error will be hidden from the end-user and the execution continues. With this change execution fails if flags.Parse() returns an error.

Same issue applies to many existing plugin implementations, such as:

• https://github.com/hashicorp/vault-auth-plugin-example/pull/32
• https://github.com/hashicorp/vault-plugin-auth-kubernetes/pull/202