cmd/juno: add filepath support for p2p CLI

[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: <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

@@            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     

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[weiihann]

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