Feat: Support non-file input for `acorn image sign --key`

[iwilltry42]

That's required e.g. for

• the acorn-io/actions-sign GitHub Action (or other scenarios), where we don't want to (temporarily) write the private key repo secret to disk
• the use cases, where the private key is not stored in a file, but rather retrieved via external tooling or some credential manager (e.g. Vault)

Example: This will work: acorn image sign --key="$(cat my.key)" my.reg.io/foo:v1

[sangee2004]

Tested with acorn version - v0.9.0-rc1-22-g08c68cf2+08c68cf2

I get "✗ ERROR: failed to import private key: invalid key file" error when trying to use the key in "$(cat sangeegh1)" format.

%acorn image sign sangeetha/testsignature:v1 --key="$(cat sangeegh1)" -a qatag=ok  
  •  Signing Image sangeetha/testsignature:v1 (digest: index.docker.io/sangeetha/testsignature@sha256:e7b2e6a60916544e2f58cd27591332f8d7132240678983057c80df00ed7e0ddd)
? Enter password for private key: *********
  ✗  ERROR:  failed to import private key: invalid key file

%acorn image sign sangeetha/testsignature:v1 --key "$(cat sangeegh1)" -a qatag=ok 
  •  Signing Image sangeetha/testsignature:v1 (digest: index.docker.io/sangeetha/testsignature@sha256:e7b2e6a60916544e2f58cd27591332f8d7132240678983057c80df00ed7e0ddd)
? Enter password for private key: *********
  ✗  ERROR:  failed to import private key: invalid key file

 % acorn image sign sangeetha/testsignature:v1 --key sangeegh1 -a qatag=ok 
  •  Signing Image sangeetha/testsignature:v1 (digest: index.docker.io/sangeetha/testsignature@sha256:e7b2e6a60916544e2f58cd27591332f8d7132240678983057c80df00ed7e0ddd)
? Enter password for private key: *********
  ✔  Created signature sha256:f084070580ffefaa634961d17126fba200b78a759a4a1838b90955c4fba06ebb

[iwilltry42]

@sangee2004 what kind of key is that? Can you paste a redacted output of cat sangeegh1 or send it via Slack, please?