Add port MRU attribute

[AnandhiDhanabalan]

Summary
Adds two port attributes that allow the maximum receive frame size (MRU) to be configured independently of SAI_PORT_ATTR_MTU. Today SAI_PORT_ATTR_MTU governs the egress frame-size limit, and on many implementations the same value is applied as the ingress admission threshold. This change lets an implementation that supports separate RX/TX enforcement expose an independent receive limit, while leaving single-threshold implementations unaffected.

Backward compatibility
SAI_PORT_ATTR_MRU takes effect only when SAI_PORT_ATTR_MRU_ENABLED is true. The default is false, under which receive-side behavior is unchanged: implementations that apply SAI_PORT_ATTR_MTU to both directions continue to do so. Existing applications require no change.

New Attributes

• SAI_PORT_ATTR_MRU_ENABLED
• SAI_PORT_ATTR_MRU

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