Support "x-ms-format": "duration-constant"

[pshao25]

Some swagger will have "x-ms-format": "duration-constant" on their model property for example. We may need a new scalar for it. What's more, TCGC needs to pass this value to emitter.

BTW I have some trial just FYI
1.

model Test {
  @format("duration-constant")
  input: duration;
}

This gives error Cannot apply @format decorator to type it is not a string

model Test {
  @format("duration-constant")
  input: string;
}

This gives warning 'string' format 'duration-constant' is not supported in Autorest. It will not be emitted. The emitted swagger does not emit this either.

More information added: For swagger input, it must be "x-ms-format": "duration-constant" for the generated swagger, otherwise this error:

fatal   | System.InvalidOperationException: The MaximumDuration property of VirtualMachineInstallPatchesParameters ends with "Duration" but does not use the duration format to be generated
   as TimeSpan type. Add "format": "duration" with directive in autorest.md for the property if it's ISO 8601 format like P1DT2H59M59S. Add "x-ms-format": "duration-constant" if it's the con 
  stant format like 1.2:59:59.5000000. If the property does not conform to a TimeSpan format, please use "x-ms-client-name" to rename the property for the client.