[mle] implement alternate RLOC16 usage during role transition

[abtink]

This commit introduces a mechanism for a device transitioning from child to router role to keep receiving frames addressed to its previous short address for a brief period.

A new radio platform API, otPlatRadioSetAlternateShortAddress(), is introduced. This allows the OT stack to configure an alternate short address. Radio platform support for this function is indicated by the OT_RADIO_CAPS_ALT_SHORT_ADDR capability in otPlatRadioGetCaps() The same function can be used with OT_RADIO_INVALID_SHORT_ADDR (0xfffe) to clear a previously set alternate short address. Support for the new API is implemented in RCP and RadioSpinel, ensuring backward compatibility by dynamically checking supported radio capabilities.

MLE code is updated to instruct the radio to use the old child RLOC16 as an alternate address upon role transition. The MLE layer will automatically clear the alternate address after eight seconds, or if other state/mode changes occur. This eight-second window ensures the new router can transmit four MLE Advertisement messages.

---

Rebased this PR on top of the commit from the following PR (used by the test).
- #10033

[size-report]

Size Report of OpenThread

Merging #10006 into main(3bf281d).

name	branch	text	data	bss	total
ot-cli-ftd	main	466840	856	66364	534060
	#10006	467240	856	66364	534460
	+/-	+400	0	0	+400
ot-ncp-ftd	main	435756	760	61576	498092
	#10006	436180	760	61576	498516
	+/-	+424	0	0	+424
libopenthread-ftd.a	main	235928	95	40310	276333
	#10006	236306	95	40310	276711
	+/-	+378	0	0	+378
libopenthread-cli-ftd.a	main	57522	0	8075	65597
	#10006	57575	0	8075	65650
	+/-	+53	0	0	+53
libopenthread-ncp-ftd.a	main	31863	0	5916	37779
	#10006	31905	0	5916	37821
	+/-	+42	0	0	+42
ot-cli-mtd	main	364488	760	51220	416468
	#10006	364568	760	51220	416548
	+/-	+80	0	0	+80
ot-ncp-mtd	main	347036	760	46448	394244
	#10006	347164	760	46448	394372
	+/-	+128	0	0	+128
libopenthread-mtd.a	main	157999	0	25182	183181
	#10006	158089	0	25182	183271
	+/-	+90	0	0	+90
libopenthread-cli-mtd.a	main	39753	0	8059	47812
	#10006	39806	0	8059	47865
	+/-	+53	0	0	+53
libopenthread-ncp-mtd.a	main	24743	0	5916	30659
	#10006	24785	0	5916	30701
	+/-	+42	0	0	+42
ot-cli-ftd-br	main	549944	864	131212	682020
	#10006	550360	864	131212	682436
	+/-	+416	0	0	+416
libopenthread-ftd-br.a	main	324242	100	105134	429476
	#10006	324646	100	105134	429880
	+/-	+404	0	0	+404
libopenthread-cli-ftd-br.a	main	71185	0	8099	79284
	#10006	71238	0	8099	79337
	+/-	+53	0	0	+53
ot-rcp	main	62248	564	20604	83416
	#10006	62328	564	20604	83496
	+/-	+80	0	0	+80
libopenthread-rcp.a	main	9542	0	5052	14594
	#10006	9584	0	5052	14636
	+/-	+42	0	0	+42
libopenthread-radio.a	main	18870	0	214	19084
	#10006	18924	0	214	19138
	+/-	+54	0	0	+54

[LuDuda]

All looks good! 👍

How do you plan to test this feature?

[abtink]

How do you plan to test this feature?

Thanks @LuDuda. I added a test that covers the basic behavior of this, i.e. the stack sets the "alt short address" after transition from child to router and after the timeout removes it.