sentry: creating a new secp256k1 context for every operation is not efficient

In sentry the secp256k1 is created for every operation which requires it (see the example below). This is not efficient, only single global context is enough.

```cpp
Bytes sign(ByteView data_hash, ByteView private_key) {
SecP256K1Context ctx{/* allow_verify = */ false, /* allow_sign = */ true};
secp256k1_ecdsa_signature signature;
bool ok = ctx.sign(&signature, data_hash, private_key);
if (!ok) {
throw std::runtime_error("ecdsa_signature::sign failed");
}

return ctx.serialize_signature(&signature);
}
```