Implement import of existing keys

[Foxboron]

λ ssh-tpm-agent master Ɇ » ssh-keygen -t ecdsa
Generating public/private ecdsa key pair.
Enter file in which to save the key (/home/fox/.ssh/id_ecdsa): ./test-key
Enter passphrase (empty for no passphrase):
Enter same passphrase again:
Your identification has been saved in ./test-key
Your public key has been saved in ./test-key.pub
The key fingerprint is:
SHA256:ZAR5B+p10hjevO9gD1X6bwOnmlPIRwLYJbvklDzop44 fox@framework
The key's randomart image is:
+---[ECDSA 256]---+
|      .o*o..     |
|      .=+O=      |
|      .oBO*   .  |
|     ..++ooo +   |
|      ..S+o *    |
|        o  = = . |
|       .  + + =  |
|      o  . *.. o.|
|     E .   o=  .o|
+----[SHA256]-----+

λ ssh-tpm-agent master Ɇ » ./ssh-tpm-keygen --import ./test-key
Sealing an existing public/private ecdsa key pair.
Enter existing password (empty for no pin):
./test-key.tpm already exists.
Overwrite (y/n)?y
Enter pin (empty for no pin):
Confirm pin:
Your identification has been saved in ./test-key.tpm
The key fingerprint is:
SHA256:ZAR5B+p10hjevO9gD1X6bwOnmlPIRwLYJbvklDzop44
The key's randomart image is the color of television, tuned to a dead channel.

Depends on google/go-tpm#341