Implement NSSecureCoding on MTRErrorHolder.

[bzbarsky-apple]

Otherwise the errors we create cannot be usefully encoded/decoded.

Fixes #22388

[github-actions]

PR #25179: Size comparison from 738205d to e15ab82

Increases (1 build for cc32xx)

platform	target	config	section	738205d	e15ab82	change	% change
cc32xx	lock	CC3235SF_LAUNCHXL	.debug_info	20360343	20360344	1	0.0

Full report (1 build for cc32xx)

platform	target	config	section	738205d	e15ab82	change	% change
cc32xx	lock	CC3235SF_LAUNCHXL		0	0	0	0.0
			(read only)	642217	642217	0	0.0
			(read/write)	203696	203696	0	0.0
			.ARM.attributes	44	44	0	0.0
			.ARM.exidx	8	8	0	0.0
			.bss	197096	197096	0	0.0
			.comment	194	194	0	0.0
			.data	1480	1480	0	0.0
			.debug_abbrev	934242	934242	0	0.0
			.debug_aranges	87448	87448	0	0.0
			.debug_frame	300212	300212	0	0.0
			.debug_info	20360343	20360344	1	0.0
			.debug_line	2661062	2661062	0	0.0
			.debug_loc	2797107	2797107	0	0.0
			.debug_ranges	281744	281744	0	0.0
			.debug_str	3019502	3019502	0	0.0
			.ramVecs	780	780	0	0.0
			.resetVecs	64	64	0	0.0
			.rodata	105705	105705	0	0.0
			.shstrtab	232	232	0	0.0
			.stab	204	204	0	0.0
			.stabstr	441	441	0	0.0
			.stack	2048	2048	0	0.0
			.strtab	377379	377379	0	0.0
			.symtab	256048	256048	0	0.0
			.text	534392	534392	0	0.0

[bzbarsky-apple]

Going to take a different approach.