additional input validation for Get-AzurermProviderOperation cmdlet

[surabhi-pandey]

No description provided.

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[ogail]

Consider renaming OperationSearchString to SearchString

[surabhi-pandey]

We decided to not use the term SearchString as the other cmdlets are using this parameter for Prefix-match or search by display name.
In our scenario, we expect the search string to be the Operation Id with optional * wildcard character(s). We don't support prefix match or search by name.

The output from this cmdlet lists following properties for each object: Operation, OperationName,OperationDescription etc.
The value for parameter OperationSearchString should correspond to the Operation property in the output.