Add eigenlayer operator keys export command

[shrimalmadhur]

Is your feature request related to a problem? Please describe.
Currently if I create a new ecdsa/bls key using eigenlayer operator keys create command, I am only shown the private key at that time. If I forget that, there's no way to get private key again.

Describe the solution you'd like
Implement an eigenlayer operator keys export command which can take a keyname and then export the private key to user. It will also ask for password and a double confirmation to continue.
We also want a flag where a user can provide a file location and the command can ignore default location and use that file location to decrypt key.

Describe alternatives you've considered
A clear and concise description of any alternative solutions or features you've considered.

Additional context
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