Added build flag to allow/skip building Example Credential files

[sharadb-amazon]

Related to #25388

Change summary

This change adds a build flag chip_build_example_creds that will allow an example app to skip building Example Credential classes (by setting it to false). Note: this flag will be set to false in the examples/tv-casting-app in a separate PR to follow.

Testing

Built and ran example/tv-casting-app successfully.

[github-actions]

PR #25953: Size comparison from 75f8ddf to 99e5941

Increases (1 build for cc32xx)

platform	target	config	section	75f8ddf	99e5941	change	% change
cc32xx	lock	CC3235SF_LAUNCHXL	.debug_info	20310415	20310416	1	0.0

Decreases (1 build for qpg)

platform	target	config	section	75f8ddf	99e5941	change	% change
qpg	lighting-app	qpg6105+debug	(read/write)	1153992	1153984	-8	-0.0
			.text	601088	601080	-8	-0.0

Full report (5 builds for cc32xx, linux, qpg)

platform	target	config	section	75f8ddf	99e5941	change	% change
cc32xx	lock	CC3235SF_LAUNCHXL		0	0	0	0.0
			(read only)	645841	645841	0	0.0
			(read/write)	203848	203848	0	0.0
			.ARM.attributes	44	44	0	0.0
			.ARM.exidx	8	8	0	0.0
			.bss	197248	197248	0	0.0
			.comment	194	194	0	0.0
			.data	1480	1480	0	0.0
			.debug_abbrev	933130	933130	0	0.0
			.debug_aranges	87712	87712	0	0.0
			.debug_frame	301608	301608	0	0.0
			.debug_info	20310415	20310416	1	0.0
			.debug_line	2681155	2681155	0	0.0
			.debug_loc	2827633	2827633	0	0.0
			.debug_ranges	286456	286456	0	0.0
			.debug_str	3041166	3041166	0	0.0
			.ramVecs	780	780	0	0.0
			.resetVecs	64	64	0	0.0
			.rodata	105953	105953	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	380565	380565	0	0.0
			.symtab	257504	257504	0	0.0
			.text	537768	537768	0	0.0
linux	chip-tool-ipv6only	arm64	(read only)	12129740	12129740	0	0.0
			(read/write)	742616	742616	0	0.0
			.bss	34392	34392	0	0.0
			.data	3008	3008	0	0.0
			.data.rel.ro	684496	684496	0	0.0
			.dynamic	560	560	0	0.0
			.got	15504	15504	0	0.0
			.init	24	24	0	0.0
			.init_array	216	216	0	0.0
			.rodata	582204	582204	0	0.0
			.text	9785268	9785268	0	0.0
	thermostat-no-ble	arm64	(read only)	2519972	2519972	0	0.0
			(read/write)	145208	145208	0	0.0
			.bss	56344	56344	0	0.0
			.data	1784	1784	0	0.0
			.data.rel.ro	77672	77672	0	0.0
			.dynamic	560	560	0	0.0
			.got	5360	5360	0	0.0
			.init	24	24	0	0.0
			.init_array	432	432	0	0.0
			.rodata	147312	147312	0	0.0
			.text	2110320	2110320	0	0.0
qpg	lighting-app	qpg6105+debug	(read/write)	1153992	1153984	-8	-0.0
			.bss	97404	97404	0	0.0
			.data	852	852	0	0.0
			.text	601088	601080	-8	-0.0
	lock-app	qpg6105+debug	(read/write)	1122544	1122544	0	0.0
			.bss	92540	92540	0	0.0
			.data	856	856	0	0.0
			.text	569644	569644	0	0.0