Create central key-loading function

Everytime you want to sign a transaction or any other qubic package, the seed is required.

The seed is encrrypted by the private.key. In order to be able to decrypt the seed, the user must provide the private.key.

This is used in several place. (e.g. voting, assets, payments).

therefore, for signing of packages, we should have  a central handler/service which does this.