feat: add Cred feature with recipe pattern and replace Credential DTO in remote options

[racgoo]

cred feature in #94

@seokju-na I wasn't assigned to this issue yet, but I wanted to give it a try. Please let me know if the approach looks good or if anything needs to be changed.

Summary

• Adds Cred class with factory methods (default(), sshKeyFromAgent(), sshKey(), sshKeyFromMemory(), userpassPlaintext(), username(), credentialHelper()) and CredType enum.
• git2::Cred does not implement Clone or Send, so it cannot be reused across callbacks or moved into async tasks. Instead, Cred stores only the parameters needed to build a git2::Cred (recipe pattern). A fresh git2::Cred is created from the recipe each time it is needed, making Cred freely cloneable and safe to send across threads.
• Removes the Credential DTO and CredentialType enum from remote.rs.
  FetchOptions, PushOptions, and PruneOptions now take Cred directly in their credential field. Credential was a flat object that held all possible fields at once (e.g. privateKeyPath was present even for a plain username/password credential), which made it unclear which fields were actually required for each credential type.
• All factory methods are infallible and return Cred directly. Errors only surface when the credential is actually used (e.g. during network I/O or hasUsername()).

Breaking changes

Before	After
FetchOptions.credential: Credential | null	FetchOptions.credential: Cred | null
PushOptions.credential: Credential | null	PushOptions.credential: Cred | null
Factory methods throw on failure	Factory methods never throw

[vercel]

@racgoo is attempting to deploy a commit to the Toss Team on Vercel.

A member of the Team first needs to authorize it.