Support generating ApiKey with prefix

[lmazuel]

This TypeSpec:

@useAuth({
  type: AuthType.http,
  scheme: "SharedAccessKey",
})

Means to authenticate using the headers:

Authorization: SharedAccessKey <put my key here>

TypeScript Azure-Core do not provide support for this, see additional discussion here:
Azure/azure-sdk-for-js#25683

Work item is to recognize the TypeSpec, and configure the AzureKeyCredentialPolicy with a prefix.

Cadl-Ranch test:
Azure/cadl-ranch#262

[lirenhe]

Only used in one service, will consider move the logic to Azure.Core if this is a common pattern.