[Fixes #201] Implement SldFileHandler

[mattiagiupponi]

ref #201

Refactor metadata common and add support for SLD upload.

PS: the PR rely on the implementation of #200 Please review #202 before this

[gitguardian]

⚠️ GitGuardian has uncovered 3 secrets following the scan of your pull request.

Please consider investigating the findings and remediating the incidents. Failure to do so may lead to compromising the associated services or software components.

🔎 Detected hardcoded secrets in your pull request

GitGuardian id	GitGuardian status	Secret	Commit	Filename
59204	Triggered	Django Secret Key	369fce9	.env_test	View secret
9468526	Triggered	Generic Password	369fce9	entrypoint_test.sh	View secret
9468526	Triggered	Generic Password	369fce9	.env_test	View secret

🛠 Guidelines to remediate hardcoded secrets

1. Understand the implications of revoking this secret by investigating where it is used in your code.
2. Replace and store your secrets safely. Learn here the best practices.
3. Revoke and rotate these secrets.
4. If possible, rewrite git history. Rewriting git history is not a trivial act. You might completely break other contributing developers' workflow and you risk accidentally deleting legitimate data.

To avoid such incidents in the future consider

• following these best practices for managing and storing secrets including API keys and other credentials
• install secret detection on pre-commit to catch secret before it leaves your machine and ease remediation.

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