Add S3Client objectExists support in S3Template

[Ziemowit]

📢 Type of change

• Bugfix
• New feature
• Enhancement
• Refactoring

📜 Description

Introduction of S3Operations#documentExists, the complementary operation to S3Operations#bucketExists.

💡 Motivation and Context

It is useful when we want to check if document exists without downloading it.

💚 How did you test it?

📝 Checklist

• I reviewed submitted code
• I added tests to verify changes
• I updated reference documentation to reflect the change
• All tests passing
• No breaking changes

🔮 Next steps

[maciejwalkowiak]

Thanks @Ziemowit !