Add amount parameter for deposit

[remyroy]

Forwarded from:

• Add amount parameter ethereum/staking-deposit-cli#326

This is potentially confusing and there should be a very conservative approach to take and implement this.

[yorickdowne]

Original requester says partial-deposit meets their needs. Makes sense to me. This is an advanced workflow and so reasonably:

• Standard workflow does 32 ETH; good UX
• Advanced users can then then partial-deposit the amount they want and use that file

Waiting for confirmation, and, potentially this is solved by Valefar’s new command.

[remyroy]

#89 should partially resolve this.

We have too many concerns modifying this on the new-mnemonic.