* Feat: move HMAC verification into enclave

- add gen_hmac_safe() function in functions.js
- add EH_GEN_HMAC action and ffi_generateHmac handler
- append GEN_HMAC to the tail of action list instead of inserting to the middle,
  so that the id of other items in the list will keep unchanged
- add enclave_generate_hmac function to enclave part
- add param verification to enclave_generate_hmac function
- add constant EH_HMAC_SHA256_SIZE as macro

* Chore: correct function description of GenerateHmac

* fix: cmk and apikey params is not checked in ffi_generateHmac

* fix: `hmac` is not free in ffi_generateHmac

* fix: clear sensitive info before return in enclave_generate_hmac

* Feat: change `payload` in ffi_generateHmac to base64 encoded string

* Fix: ehsm_kms_service/function.js still using raw string

- optimize the implementation of decoding `payload` params in ffi_generateHmac

* refactor: pass NULL to aad when decrypting cmk in enclaveGenHmac

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Co-authored-by: Tsz Lung, Chung <tsz.lung.chung@intel.com>