Inject local keys as authorities and backup old ones

[mustermeiszer]

For a lot of actions, especially between relay-chain and parachain there is signing involved from the current block authoring entity. E.g. the bitfields must be signed by the authority of this block.

In order to actually create valid signatures each Builder structure must expose some functionitliy to:

• Replace all authorities for block production and validation with the builders key
• Store the old authorities in the builder somewhere
• Restore old authorities state if wanted via methods

Builder struct tasks:

• [ ] Build builder with keystore and a given pub-key to be sign stuff with
• [ ] Expose method to switch authorities
• [ ] Expose method to restore old-authorities