[crypto] Fix compile error in BufferWriter init

[tima-q]

Issue:
In CHIPCryptoPALPSA.cpp

BufferWriter class expects a (uint8_t*, size_t) as initialization. The given parameter output was a P256SerializedKeypair which throws a compiler error.

Output: used compiler - gcc-arm-none-eabi-9-2019-q4-major

In member function 'virtual CHIP_ERROR chip::Crypto::P256Keypair::Serialize(chip::Crypto::P256SerializedKeypair&) const': .../src/crypto/CHIPCryptoPALPSA.cpp:685:51: 
error: no matching function for call to 'chip::Encoding::BufferWriter::BufferWriter(chip::Crypto::P256SerializedKeypair&, const size_t&)'
  685 |     Encoding::BufferWriter bbuf(output, outputSize);

Fix:
Use the Bytes method to expose a uint8_t* for use of the BufferWriter.

[github-actions]

PR #25015: Size comparison from bbfd5d6 to c30660f

Full report (2 builds for cc32xx, mbed)

platform	target	config	section	bbfd5d6	c30660f	change	% change
cc32xx	lock	CC3235SF_LAUNCHXL		0	0	0	0.0
			(read only)	640233	640233	0	0.0
			(read/write)	204084	204084	0	0.0
			.ARM.attributes	44	44	0	0.0
			.ARM.exidx	8	8	0	0.0
			.bss	197488	197488	0	0.0
			.comment	194	194	0	0.0
			.data	1476	1476	0	0.0
			.debug_abbrev	928439	928439	0	0.0
			.debug_aranges	87352	87352	0	0.0
			.debug_frame	299840	299840	0	0.0
			.debug_info	20194444	20194444	0	0.0
			.debug_line	2649825	2649825	0	0.0
			.debug_loc	2785969	2785969	0	0.0
			.debug_ranges	280696	280696	0	0.0
			.debug_str	3001474	3001474	0	0.0
			.ramVecs	780	780	0	0.0
			.resetVecs	64	64	0	0.0
			.rodata	105585	105585	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	375840	375840	0	0.0
			.symtab	255856	255856	0	0.0
			.text	532524	532524	0	0.0
mbed	lock-app	CY8CPROTO_062_4343W+release	(read only)	6224	6224	0	0.0
			(read/write)	2464576	2464576	0	0.0
			.bss	215892	215892	0	0.0
			.data	5880	5880	0	0.0
			.text	1427220	1427220	0	0.0