Add OpenAPI Go and C#

[yhyakuna]

Vault 1.13 Deliverable

🎫 Jira
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Update the Developer Quick Start page by adding new tabs in the Install Client Library, Authenticate to Vault, Store a secret & Retrieve a Secret for "OpenAPI Go (Beta)", and "OpenAPI C# (Beta)"

What's missing:

• No example code in the vault-examples repo for OpenAPI Go and OpenAPI C#.

• No code example equivalent to:

  if value != "Hashi123" {
      log.Fatalf("unexpected password value %q retrieved from vault", value)
  }
  

---

NOTE:
Had to switch <CodeTabs> to <Tabs> since the tab heading can NOT be customized using <CodeTabs>.

[averche]

This is great! Thank you for adding these! 🎉

Could you please also add links to the following two under "The complete code samples for the steps below are available here:"

https://github.com/hashicorp/vault-client-go/#getting-started
https://github.com/hashicorp/vault-client-dotnet/#getting-started

[averche]

This is great, thank you 🎉

[AnPucel]

LGTM! Thanks for doing this!