Refactor KeyPair raw byte handling

[davidatwhiletrue]

Summary

Refactors KeyPair creation and serialization behavior around raw private-key bytes.

• Adds KeyPair.Create(...) while keeping CreateNew(...) as an obsolete compatibility wrapper.
• Adds KeyPair.FromBytes(...) and RawBytes for private-key byte round trips.
• Derives public keys from private key material instead of storing a separate public key parameter.
• Writes public-key PEMs through PublicKey.WriteToPem(...).
• Normalizes secp256k1 signatures without producing intermittently invalid signatures.
• Enforces 32-byte private-key input for both ED25519 and SECP256K1.
• Updates internal tests/docs to use KeyPair.Create(...) instead of CreateNew(...).

Validation

• dotnet test Casper.Network.SDK.Test/Casper.Network.SDK.Test.csproj --no-restore --filter "FullyQualifiedName~NetCasperTest.KeysTest"
• dotnet build Casper.Network.SDK.Test/Casper.Network.SDK.Test.csproj --no-restore
• dotnet build Casper.Network.SDK/Casper.Network.SDK.csproj --no-restore
• rg "KeyPair\\.CreateNew" -n

Note: full git diff --check still reports an unrelated pre-existing trailing whitespace line in Casper.Network.SDK.Test/NctlQueryGlobalStateTest.cs:27, outside the keypair branch scope.

[alexmyshchyshyn]

LGTM