[crypto] Fix CASE on some embedded platforms

[Damian-Nordic]

Problem

Establishing a CASE session requires support for P256Keypair::ECDH_derive_secret() function which was
disabled for platforms which define MBEDTLS_ECP_ALT. It is quite common on embedded devices equipped with
dedicated crypto chips to provide custom ECP implementation. As a result, an attempt to resolve a device address or send a ZCL message would fail for nRF Connect-based accessories (other platforms such as Qorvo are probably affected, too).

Change overview

Unblock P256Keypair::ECDH_derive_secret() for platforms which provide a custom ECP implementation.

Testing

Tested commissioning + ZCL commands using the Python controller and nRF Connect Lock Example.

[todo]

Enable ECDH_derive_secret for Qorvo when their mbedTLS static libraries are updated

connectedhomeip/src/crypto/CHIPCryptoPALmbedTLS.cpp

Lines 562 to 566 in e0a3388

	// TODO: Enable ECDH_derive_secret for Qorvo when their mbedTLS static libraries are updated
	#if defined(MBEDTLS_ECDH_C) && !defined(QORVO_CRYPTO_ENGINE)
	CHIP_ERROR error = CHIP_NO_ERROR;
	int result = 0;
	size_t secret_length = (out_secret.Length() == 0) ? out_secret.Capacity() : out_secret.Length();

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This comment was generated by todo based on a TODO comment in e0a3388 in #7948. cc @Damian-Nordic.