Add ClientInitialization usage for models

[m-nash]

Clear and concise description of the problem

Right now getAllModels does not include models which have usage as none. For the client initialization model the usage is none since it isn't used in any of the service operations. We should introduce a new usage flag and apply it to the client initialization model so that it is returned by getAllModels.

Emitters who don't do anything with this model can easily ignore it by looking for this usage flag.

Checklist

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[tadelesh]

with this #2027, tcgc deprecate the SdkInitializationType type and add SdkClientInitializationMethod. the options model should be language specific, so tcgc does not add that anymore.