Add command to create Era keypair in aggregator

[jpraynaud]

Why

When setting up a test aggregator we may need to create test era keys and there is no command available on the aggregator for doing this.

A similar command has already been added to the genesis command (see #2074)

What

Add a new sub-command to the era command to create the non deterministic era keypair.

How

• Create a new sub-command generate-keypair for the era command
  • by using the EraMarkers::create_non_deterministic_signer
  • with a custom export directory
  • by exporting a era.sk and era.vk files in the export directory
• Update documentation