Describe the desired outcome from the user's perspective
As a user configuring the Schema Registry connection in zilla.yaml, I want to provide HTTP Basic credentials (username and password) in plain form directly or through environment variables.

Reference:

schema-registry:
  type: schema-registry
  options:    
    url: ${{env.SCHEMA_REGISTRY_URL}}
    context: default
    credentials:
      basic: 
        username: ${{env.SCHEMA_REGISTRY_BASIC_AUTH_USERNAME}}
        password: ${{env.SCHEMA_REGISTRY_BASIC_AUTH_PASSWORD}}

Currently, zilla supports Basic token or Bearer token.

Ref: https://docs.aklivity.io/zilla/latest/reference/config/catalogs/schema-registry.html#options-credentials

Acceptance criteria

• Allow users to configure username and password in plain form in zilla.yaml.