feat(snap-networks-utils): add shareable keyring account helpers

[Julink-eth]

Explanation

• Add ExtendedKeyringAccount, KeyringAccountExtension, and asStrictKeyringAccount to @metamask/snap-networks-utils.
• The three snaps all extend @metamask/keyring-api's KeyringAccount with the same fields (entropySource, derivationPath, index) and the same strict-conversion helper (Solana/Tron asStrictKeyringAccount, Stellar #toKeyringAccount).
• A snap may still narrow derivationPath (Stellar's BIP-44 coin type). @metamask/keyring-api@^24.1.0 is added as a type dependency so those exports compile and match the snaps.
  Not client-breaking: new helpers only.

References

• Ticket: WPN-1639

Checklist

• I've updated the test suite for new or updated code as appropriate
• I've updated documentation (JSDoc, Markdown, etc.) for new or updated code as appropriate
• I've communicated my changes to consumers by updating changelogs for packages I've changed
• I've introduced breaking changes in this PR and have prepared draft pull requests for clients and consumer packages to resolve them

[sonarqubecloud]

Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
100.0% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarQube Cloud