feat: Add auxiliary token policy and update VMsClient to use in client options

[rakechill]

Description
This PR adds:

• GetAuxiliaryToken() which will return a token that can authenticate with shared node image galleries for managed Karpenter users.
• AuxiliaryTokenPolicy which provides a policy to add an auxiliary token to a request
• Updates VirtualMachinesClient to use this policy in its opts

How was this change tested?

• [WIP] e2e testing environment with msi-connector dependency
• TODO: unit tests

Does this change impact docs?

• Yes, PR includes docs updates
• Yes, issue opened: #
• No

Release Note

[coveralls]

Pull Request Test Coverage Report for Build 11509167009

Details

• 0 of 54 (0.0%) changed or added relevant lines in 3 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage decreased (-0.1%) to 97.44%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
pkg/auth/policy.go	0	7	0.0%
pkg/providers/instance/azure_client.go	0	19	0.0%
pkg/auth/cred.go	0	28	0.0%

Totals
Change from base Build 11508710261:	-0.1%
Covered Lines:	36994
Relevant Lines:	37966

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💛 - Coveralls