[FEA][accounts] Support multiple accounts in self-managed NVCF

[sbaum1994]

Is your feature request related to a problem? Please describe.

Current self-managed installation flows default to a single nvcf-default account. Operators need a supported way to create and manage multiple accounts for tenant, environment, or team separation.

Without multi-account support, operators must either share one account across users and workloads or rely on unsupported manual setup.

Describe the solution you'd like

Add supported multiple-account management for self-managed NVCF.

The implementation should cover:

• Account creation and bootstrap beyond nvcf-default.
• CLI account selection and configuration.
• API key issuance scoped to the intended account.
• Cluster registration against the intended account.
• Function create, deploy, invoke, and list behavior per account.
• Docs for account lifecycle and operational limits.

Describe alternatives you've considered

Operators can deploy separate NVCF control planes for isolation, but that increases operational cost and does not address account-level workflows inside a single deployment.

Additional context

Suggested acceptance criteria:

• More than one account can be created in a self-managed deployment.
• API keys and scopes are account-specific.
• Cluster registration and function lifecycle calls work for non-default accounts.
• Docs explain account setup, selection, and cleanup.
• Tests cover at least two accounts in the same control plane.