feat: add secrets management

[LucaLanziani]

No description provided.

[mablanco]

I've tested the code and I think we're good to go on merging:

$  go run main.go secrets
NAME:
   initium secrets - A series of command to generate age keys, encrypt and decrypt secrets

USAGE:
   initium secrets command [command options] [arguments...]

COMMANDS:
   generate-keys  Generate the public and private keys and output them on stdout
   encrypt        Encrypt a secret, if the secret flag is used the secret is first encoded in base64 and then encrypted
   decrypt        Decrypt a base64 encoded secret and output the base64 encoded value
   help, h        Shows a list of commands or help for one command

OPTIONS:
   --help, -h  show help
$ go run main.go secrets encrypt
2023/11/16 08:40:33 FATA required flags "public-key, plain-secret, base64-plain-secret" not set
exit status 1
$ go run main.go secrets encrypt -h
NAME:
   initium secrets encrypt - Encrypt a secret, if the secret flag is used the secret is first encoded in base64 and then encrypted

USAGE:
   initium secrets encrypt [command options] [arguments...]

OPTIONS:
   --public-key value            [$INITIUM_SECRET_PUBLIC_KEY]
   --plain-secret value
   --base64-plain-secret value
   --help, -h                   show help