[Light client] Optimize signature verification

[ijerkovic]

Once NEP-364 is ready, we should use the ed25519_verify host function for verifying signatures in the add_light_client_block method. The new signature verify method should be significantly more efficient since it is part of NEAR runtime as a pre-compiled function.

Original NEP:
near/NEPs#364

Tracking issue for NEP-364:
near/nearcore#7567