Feature Request: Provide vtsql credentials to VTAdmin via flag input

[notfelineit]

Feature Description

We'd like to be able to provide vtsql credentials to VTAdmin directly via flag input, as opposed to providing the credentials in a file via vtsql_credentials_path_tmpl.

Use Case(s)

This is useful when using dynamic cluster discovery, where it's not as easy to dynamically create credential files and pass it to VTAdmin API directly, especially in a distributed system where VTAdmin API may be deployed separately from VTAdmin web and different authorization systems are used in each component.

Anywhere where it's not convenient to create and pass a credentials file, direct flag values may be used.