Add method to import keys into keychain

[rendaw]

Per ipfs/kubo#4240 I thought I'd move toward the first steps here.

Exported key storage format

• This can be an opaque format. The goal is backing up one's identity and using it on multiple workstations/devices, neither of which usecases need to know the internal format.
• PrivKey already has marshal/unmarshal. I don't know if these include a version number/are future proof/are frozen but PrivKey is a good target since everything else can be derived from it.

Import
Since the serialized format is opaque Import takes a blob.

Export
Users might want to install ipfs, generate a key, and import it on another (remote?) machine without setting up ipfs locally. That said, I think there are two options here:

1. Add additional parameters to Generate (flag to generate only without storing), plus the private key as a blob would need to be included in Key or a new return type defined. I'm not sure what backward compatibility guarantees are here... I've seen some other libraries do things like Generate2 Generate3 to avoid breaking the interface.
2. Alternatively it might make sense to ignore Generate and crypto or something can provide a standalone function for creating keys from KeyGenerateOption, which can then be added with Import. In that case only Import is needed here I think.

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[rendaw]

Two other thoughts.

1. If the format is opaque, maybe this interface should also include ascii encoding and return a string instead.
2. If the marshalled output of p2p crypto keys isn't stable, maybe going to std crypto and using those marshal functions would be an option.

[rendaw]

Closing per discussion in issue ticket.