Leverage sharded origin key

[Neylix]

Description

First software origin key is no more hardcoded but randomly generated during network init and encrypted with the hardcoded public key in secret transaction.
Hardcoded key is now able to read and decrypt first software origin key

Fixes #275

Type of change

• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• This change requires a documentation update

How Has This Been Tested?

• Implement file test to ensure that an origin key is generated during network init
• Used GraphiQL to get first origin_shared_secret transaction and decypt it using libjs. Resulted private key is the same that the one generated during network init

Checklist:

• My code follows the style guidelines of this project
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• My changes generate no new warnings
• I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes
• Any dependent changes have been merged and published in downstream modules

[Neylix]

As this PR change the way to interact with blockchain from client software, we have to provide them the way to get the origin private key before merging this PR.
We have to implement #158 and add this API in all librairies (i.e. libjs and libdart actually)

[ghost]

Looks good.
I would suggest as well to add a content formatter for the origin_shared_secrets transaction in the explorer to be able to see the public keys by parsing them.

[ghost]

Instead of putting this in the crypto.ex, you should move it to shared_secrets.ex, to avoid confusion with origin_id for public keys