Add missing functions/implementations of ExtSecretKey, ExtPubKey and DerivationPath in c-bindings

[Alesfatalis]

Current c-bindings implementation is missing some useful functions for ExtSecretKey like:

• get derivation path from ExtSecretKey
• get secret key from ExtSecretKey
• get public key from ExtSecretKey
• derive a new ExtSerectKey based on derivation path

Currently the function ext_secret_key_new in bindings/ergo-lib-c-core/src/ext_secret_key.rs accepts a string as derivation path but should be DerivationPath which needs to be implemented first since it is missing (see below).

Following types are completely missing in c-bindings:

• ExtPubKey
• DerivationPath