Add Git Credential Helper as master password source

[Copilot]

Implements support for git credential helpers as master password sources. Git credential helpers provide a standardized protocol for retrieving credentials from various backends (cache, store, OS keychains, etc.) using URLs as lookup keys.

Implementation

• GitCredentialHelperMasterSource: New master source following the PrefixMasterSourceSupport pattern
• Configuration format: git-credential:helper-name?url=protocol://host/path
  • Helper name can be short form (e.g., cache) or full path to executable
  • URL serves as artificial lookup key for credential storage
• Protocol handling: Executes helper with get action, writes URL components to stdin, parses password= field from stdout
• Validation: Checks helper availability by attempting execution with closed stdin

Usage

<!-- Use git-credential-cache -->
<masterSource>git-credential:cache?url=https://maven.apache.org/master</masterSource>

<!-- Use OS keychain on macOS -->
<masterSource>git-credential:osxkeychain?url=https://maven.apache.org/master</masterSource>

<!-- Use custom helper -->
<masterSource>git-credential:/usr/local/bin/my-helper?url=https://maven.apache.org/master</masterSource>

Store credentials using standard git credential protocol:

echo -e "protocol=https\nhost=maven.apache.org\npath=/master\npassword=yourPassword\n" | git-credential-store store

Testing

Comprehensive unit tests with mock helper execution covering configuration parsing, validation, and cross-platform compatibility.

Original prompt

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This section details on the original issue you should resolve

<issue_title>Add Git Credentials Helper as master source</issue_title>
<issue_description>There are lots of git credentials helper listed in https://git-scm.com/doc/credential-helpers.

As all have a common way of how they are parameterized and return credentials they can be added as master source as well.

As Git Credential helpers always use a URL as lookup key one would need to use an artificial URL to lookup the correct entry. In addition one would need to add the filename of the credential helper binary.
</issue_description>

Comments on the Issue (you are @copilot in this section)

• Fixes Add Git Credentials Helper as master source #111

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[kwin]

@copilot Turn into package protected static method to ease unit testing. Also adjust according unit tests to no longer rely on reflection.