[EFR32] Heap fragmentation issue cause AddOpCert request to fail

[jmartinez-silabs]

Problem

EFR32 examples fail Secure session establisment with a failure when adding OpCert

00> <info  > [ZCL] OpCreds: Failed AddOpCert request.

The error happend when trying to store the OperationCertificate for this admin, the creation of a new StorableAdminPairingInfo does a malloc of 1976 that fails.

CHIP_ERROR AdminPairingInfo::StoreIntoKVS(PersistentStorageDelegate * kvs)
{
    CHIP_ERROR err = CHIP_NO_ERROR;

    char key[KeySize()];
    ReturnErrorOnFailure(GenerateKey(mAdmin, key, sizeof(key)));

    StorableAdminPairingInfo * info = chip::Platform::New<StorableAdminPairingInfo>();
    ReturnErrorCodeIf(info == nullptr, CHIP_ERROR_NO_MEMORY);

The whole process does multiples mallocs of rather large size. #7695

After analysing the heap usage, There was enough free heap but not a big enough block for the StorableAdminPairingInfo

Change overview

Quick band AID fix: Adding 3k to the heap of efr32 examples helps with the current fragmentation issues.
Could Add more heap to the MG12 MCU family but MG21 is already starving for RAM.

TODO: refactor efr32 example to use heap4 for FreeRTOS and wrap all allocs to the freeRTOS api. Improve heap monitor and tracking to better analyse and optimize the heap usage.

Testing

Manual tests with python controller on the differents examples
connect -ble 3840 73141520 1234
confirm Secure Session to Device Established

Fixes #7390

[jmartinez-silabs]

@bzbarsky-apple @chrisdecenzo @Damian-Nordic Please can you take a look, I need one more approve so this can be merge and it is prevent example for running. Thank you.