POC: Proposed solution for flexible azure auth

[frodopwns]

Allows flexible authentication options for the Resource Group and Azure SQL operators as a way for proving the concept.

This PR adds 2 environment variables:

• AZURE_OPERATOR_AUTH_SOURCE_MODE
• AZURE_OPERATOR_AUTH_SOURCE_NAMESPACE

AZURE_OPERATOR_AUTH_SOURCE_MODE defaults to "fallback".
Potential values:

• fallback: Operators tries to find service principal in namespace where resource request is created. If not found, the operator tries to find a credential in an admin namespace if AZURE_OPERATOR_AUTH_SOURCE_NAMESPACE is set. If those fail, the operator falls back to the current default of using a global credential.
• namespace: Operator tries to find a credential named aso-auth in the same namespace as the request. This is a strict mode so no alternatives will be used.
• admin-namespace: Operator tries to find a credential in the admin namespace designated by the env var mentioned above. The credential secret in that namespace would be named aso-auth-{target_namespace}. This allows admins to store secrets that grant permissions for deployment in other namespaces. This is also a strict mode.
• global: Operator uses the current default which is to use a global service principal.

The Async Controller would source the credential before calling Ensure or Delete.

relates to #1173
relates to #1188

What this PR does / why we need it:
proposes a solution for allowing multiple service principals and later managed identities to be used in multi-tenant clusters

Special notes for your reviewer:
Look at the implementation and submit your feedback. This is a POC.

[davefellows]

Suggested change

	// AuthSourceMode determines the strategy used to soruce auth credentials when authenticating with Azure
	// AuthSourceMode determines the strategy used to source auth credentials when authenticating with Azure

[matthchr]

@babbageclunk I think as discussed we can close this PR based on the work you're doing?