Allow DATABASE config in file

[sisuresh]

Description

Extension on #5177

Checklist

• Reviewed the contributing document
• Rebased on top of master (no merge commits)
• Ran clang-format v8.0.0 (via make format or the Visual Studio extension)
• Compiles
• Ran all tests
• If change impacts performance, include supporting evidence per the performance document

[Copilot]

Pull request overview

Extends the existing “external secret” ($FILE:) resolution mechanism to support loading the DATABASE connection string from a file, aligning database configuration with other secret-handling config fields.

Changes:

• Resolve DATABASE via secretmanager::resolve() during config processing (supporting $FILE:).
• Add config tests covering $FILE:-based DATABASE and inline backward compatibility.
• Document $FILE: usage for DATABASE in the example configuration.

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated 3 comments.

File	Description
src/main/Config.cpp	Resolves DATABASE via secret manager and flags external secret usage.
src/main/test/ConfigTests.cpp	Adds tests for $FILE:-based DATABASE and inline DATABASE.
docs/stellar-core_example.cfg	Documents $FILE:-based DATABASE configuration and permission expectations.