File path support for p2p private key

[weiihann]

Description

This PR adds filepath support to two existing P2P-related tools and flags, namely genp2pkeypair and --p2p-private-key.

Example

Command: juno genp2pkeypair --file nodekey

• This would generate a file containing the key pairs information.
• Note that users can still do juno genp2pkeypair just like before.

Command: juno --p2p-private-key nodekey

• This would read the private key given in the nodekey file.
• Note that users can also just parse the private key (i.e. juno --p2p-private-key <private-key>) just like before.
• The content of the file should either be just the private key or contains the "P2P Private Key:" prefix. Examples are shown below:

  
  # Example 1: Pure private key
  <private-key>

Example 2: With prefix

P2P Private Key:

### Rationale
It's more of a UX nicety to allow users to generate key pairs in a file and parse the file to use as a private key, rather than having to copy pasting manually.

[codecov[bot]]

Codecov Report

Attention: Patch coverage is 70.00000% with 9 lines in your changes missing coverage. Please review.

Project coverage is 75.44%. Comparing base (a30d388) to head (2abdcd9). Report is 41 commits behind head on main.

Files	Patch %	Lines
cmd/juno/juno.go	53.33%	4 Missing and 3 partials :warning:
cmd/juno/genp2pkeypair.go	86.66%	1 Missing and 1 partial :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #1904 +/- ## ========================================== - Coverage 75.51% 75.44% -0.08% ========================================== Files 100 100 Lines 8982 9003 +21 ========================================== + Hits 6783 6792 +9 - Misses 1599 1605 +6 - Partials 600 606 +6 ```

:umbrella: View full report in Codecov by Sentry.
:loudspeaker: Have feedback on the report? Share it here.

[weiihann]

On a second thought, I find this feature irrelevant as users can just use environment variables.