SecretStore: ECDSA session for cases when 2*t < N

[svyatonik]

implements #7615
this PR is only about c0a8f8d commit

Background description is in the #7615

API changes

1. Schnorr signature is generated by GET-ting /schnorr/{server_key_id}/{signature}/{message_hash} (previously: GET to /{server_key_id}/{signature}/{message_hash})
2. ECDSA signature is generated by GET-ting /ecdsa/{server_key_id}/{signature}/{message_hash}

Usage example

1. create account
2. sign server key id using this account:
   curl --data-binary '{"jsonrpc": "2.0", "method": "secretstore_signRawHash", "params": ["0x00a329c0648769A73afAc7F9381E08FB43dBEA72", "", "0x0000000000000000000000000000000000000000000000000000000000000003"], "id":1 }' -H 'content-type: application/json' http://127.0.0.1:8545/
3. create server key using generated signature:
   curl -v -X POST http://localhost:8082/shadow/0000000000000000000000000000000000000000000000000000000000000003/bd22d48735e5d711fffa03d9a987366acd50359a7a27f2fb0480e5339a44bf155b36ee1f2f860f4fcbb1171cf68dd59ac5f95d407137cafec4898f3e562fa9d700/1
4. sign message hash using server key:
   curl -v http://localhost:8082/ecdsa/0000000000000000000000000000000000000000000000000000000000000003/bd22d48735e5d711fffa03d9a987366acd50359a7a27f2fb0480e5339a44bf155b36ee1f2f860f4fcbb1171cf68dd59ac5f95d407137cafec4898f3e562fa9d700/0000000000000000000000000000000000000000000000000000000000000005