fix/enhancement: age-inspect indefinitely hanging and also improvements to usage info and flags for both age-inspect and age-keygen

[Turmaxx]

This PR mainly fixes an issue where age-inspect would hang indefinitely, when invoked alone without arguments or a piped input. but also improves the usage information and version flags a bit for different age CLI binaries.

• adds a v as an alternative version flag to age-inspect and age-keygen.
• improves the usage information, by adding alternate usage to age-inspect.

In relation to age-inspect, the changes I've made wont affect previous behavior.

$ age-inspect

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Usage:
    age-inspect [INPUT]
    age-inspect [--json] [INPUT]

Options:
    --json                      Output machine-readable JSON.
    -v, --version               Print the version.

INPUT defaults to standard input. "-" may be used as INPUT to explicitly
read from standard input.

$ age-inspect test.age

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test.age is an age file, version "age-encryption.org/v1".

This file is ASCII-armored.

This file is encrypted to the following recipient types:
  - "scrypt"

This file uses post-quantum encryption.

Size breakdown (assuming it decrypts successfully):

    Header                       150 bytes
    Armor overhead              1481 bytes
    Encryption overhead           32 bytes
    Payload                     3805 bytes
                        -------------------
    Total                       5468 bytes

Tip: for machine-readable output, use --json.

$ age-inspect --json test.age

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{
    "version": "age-encryption.org/v1",
    "postquantum": "yes",
    "armor": true,
    "stanza_types": [
        "scrypt"
    ],
    "sizes": {
        "header": 150,
        "armor": 1481,
        "overhead": 32,
        "min_payload": 3805,
        "max_payload": 3805,
        "min_padding": 0,
        "max_padding": 0
    }
}

$ cat test.age | age-inspect

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<stdin> is an age file, version "age-encryption.org/v1".

This file is ASCII-armored.

This file is encrypted to the following recipient types:
  - "scrypt"

This file uses post-quantum encryption.

Size breakdown (assuming it decrypts successfully):

    Header                       150 bytes
    Armor overhead              1481 bytes
    Encryption overhead           32 bytes
    Payload                     3805 bytes
                        -------------------
    Total                       5468 bytes

Tip: for machine-readable output, use --json.

$ cat test.age | age-inspect --json

Click to expand output

{
    "version": "age-encryption.org/v1",
    "postquantum": "yes",
    "armor": true,
    "stanza_types": [
        "scrypt"
    ],
    "sizes": {
        "header": 150,
        "armor": 1481,
        "overhead": 32,
        "min_payload": 3805,
        "max_payload": 3805,
        "min_padding": 0,
        "max_padding": 0
    }
}

[Turmaxx]

You probably have to update go.mod @FiloSottile, that's why the CI failed.