[Feature]: Support importing private keys in hex format

[zakir-code]

Summary

Add support for importing private keys in hex format in <app> keys import

Problem Definition

why

• General wallets support the export of hex type private keys, but not necessarily armor.
• The user may only want to import a single private key instead of the mnemonic, and then this private key may come from a wallet such as kepler.

disadvantages

• May not be compatible with importing different types of keys(e.g. ethsecp256k1).

Proposal

[bizk]

it makes sense to translate it from hex, I'm wondering if we receive other metadata from such wallets to infer what type of key is being decoded.

We might want to export keys to hex as well

[zakir-code]

1. I think the "--key-type" flag can be added to distinguish the type of private key
2. Currently supported: simd keys export <name> --unarmored-hex --unsafe

[JulianToledano]

We might want to export keys to hex as well

Thats already possible with the export command. Flags --unsafe and --unarmored-hex are needed. Example:
./simd keys export myKeyUid --unsafe --unarmored-hex