Add OSS stub functions for Self-Managed Static Roles

[vinay-gopalan]

Description

This PR adds the Self-Managed Static Role workflow fields and mechanism, along with OSS Stub functions to ensure this feature is Enterprise-Only. These fields will exist in this repo but full functionaility is only available in Vault Ent. Please see the Vault Ent PR for further reference.

Link to ADR for feature.

Enterprise PR link: https://github.com/hashicorp/vault-enterprise/pull/6261

TODO only if you're a HashiCorp employee

• Backport Labels: If this PR is in the ENT repo and needs to be backported, backport
  to N, N-1, and N-2, using the backport/ent/x.x.x+ent labels. If this PR is in the CE repo, you should only backport to N, using the backport/x.x.x label, not the enterprise labels.
  • If this fixes a critical security vulnerability or severity 1 bug, it will also need to be backported to the current LTS versions of Vault. To ensure this, use all available enterprise labels.
• ENT Breakage: If this PR either 1) removes a public function OR 2) changes the signature
  of a public function, even if that change is in a CE file, double check that
  applying the patch for this PR to the ENT repo and running tests doesn't
  break any tests. Sometimes ENT only tests rely on public functions in CE
  files.
• Jira: If this change has an associated Jira, it's referenced either
  in the PR description, commit message, or branch name.
• RFC: If this change has an associated RFC, please link it in the description.
• ENT PR: If this change has an associated ENT PR, please link it in the
  description. Also, make sure the changelog is in this PR, not in your ENT PR.

[github-actions]

CI Results:
All Go tests succeeded! ✅

[github-actions]

Build Results:
All builds succeeded! ✅

[thyton]

Stubs addition and TestUpdateUser_SelfManaged_OSS() look great!

[Zlaticanin]

LGTM