feat: gnokey can export key for validators

[D4ryl00]

fixes #3113

Context

• The original issue has been created before gnokms; while gnokms can already use gnokey keys via remote signing, there was no direct way to emit a priv_validator_key.json from a gnokey account.
• Consensus accepts both ed25519 (default) and secp256k1 validator pubkeys, so exporting a gnokey secp256k1 key as a validator key is valid.

Changes

• Added a -validator flag to gnokey export to write a priv_validator_key.json using the selected gnokey key (secp256k1).
• Default output is priv_validator_key.json in the current directory; can override with -output-path.
• Introduced PersistFileKey helper to persist any provided private key in the validator key file format; used by the new export path.

Usage

gnokey export -validator -key <key_name> [-output-path ./priv_validator_key.json]

• If -output-path is omitted, the file is written to priv_validator_key.json in the CWD.

[Gno2D2]

🛠 PR Checks Summary

All Automated Checks passed. ✅

Manual Checks (for Reviewers):

• IGNORE the bot requirements for this PR (force green CI check)
• The pull request description provides enough details

Read More

🤖 This bot helps streamline PR reviews by verifying automated checks and providing guidance for contributors and reviewers.

✅ Automated Checks (for Contributors):

🟢 Maintainers must be able to edit this pull request (more info)
🟢 Pending initial approval by a review team member, or review from tech-staff

☑️ Contributor Actions:

1. Fix any issues flagged by automated checks.
2. Follow the Contributor Checklist to ensure your PR is ready for review.
   • Add new tests, or document why they are unnecessary.
   • Provide clear examples/screenshots, if necessary.
   • Update documentation, if required.
   • Ensure no breaking changes, or include BREAKING CHANGE notes.
   • Link related issues/PRs, where applicable.

☑️ Reviewer Actions:

1. Complete manual checks for the PR, including the guidelines and additional checks if applicable.

📚 Resources:

• Report a bug with the bot.
• View the bot’s configuration file.

Debug

Automated Checks

Maintainers must be able to edit this pull request (more info)

If

🟢 Condition met
└── 🟢 And
    ├── 🟢 The base branch matches this pattern: ^master$
    └── 🟢 The pull request was created from a fork (head branch repo: D4ryl00/gno)

Then

🟢 Requirement satisfied
└── 🟢 Maintainer can modify this pull request

Pending initial approval by a review team member, or review from tech-staff

If

🟢 Condition met
└── 🟢 And
    ├── 🟢 The base branch matches this pattern: ^master$
    └── 🟢 Not (🔴 Pull request author is a member of the team: tech-staff)

Then

🟢 Requirement satisfied
└── 🟢 If
    ├── 🟢 Condition
    │   └── 🟢 Or
    │       ├── 🟢 User jefft0 already reviewed PR 4980 with state APPROVED
    │       ├── 🔴 At least 1 user(s) of the team tech-staff reviewed pull request
    │       └── 🔴 This pull request is a draft
    └── 🟢 Then
        └── 🟢 Not (🔴 This label is applied to pull request: review/triage-pending)

Manual Checks

**IGNORE** the bot requirements for this PR (force green CI check)

If

🟢 Condition met
└── 🟢 On every pull request

Can be checked by

• Any user with comment edit permission

The pull request description provides enough details

If

🟢 Condition met
└── 🟢 And
    ├── 🟢 Not (🔴 Pull request author is a member of the team: core-contributors)
    └── 🟢 Not (🔴 Pull request author is user: dependabot[bot])

Can be checked by

• team core-contributors

[codecov]

Codecov Report

❌ Patch coverage is 84.61538% with 4 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
tm2/pkg/crypto/keys/client/export.go	77.77%	2 Missing and 2 partials ⚠️

📢 Thoughts on this report? Let us know!

[jefft0]

The PR has a resolution to the issue. Has tests. CI checks pass. I built gnokey locally. The -validator flag works as documented. Ready for core devs to review the solution.