Fills the metadata attribute, in the get NAD's output

[CBiasuzzi]

Please check if the PR fulfills these requirements

• The commit message follows our guidelines
• Tests for the changes have been added (for bug fixes / features)
• Docs have been added / updated (for bug fixes / features)

Does this PR already have an issue describing the problem?

no

What kind of change does this PR introduce?

feature

What is the current behavior?

In addition to the NAD's SVG, the NAD's metadata is used by some visualization tools developed with pypowsybl.
However, the current Network.get_network_area_diagram method returns an instance of Svg where the metadata attribute is always None.

What is the new behavior (if this is a feature change)?

The output Svg's 'metadata' attribute is now set with the NAD metadata.

Does this PR introduce a breaking change or deprecate an API?

• Yes
• No

If yes, please check if the following requirements are fulfilled

• The Breaking Change or Deprecated label has been added
• The migration steps are described in the following section

What changes might users need to make in their application due to this PR? (migration steps)

Other information:

[sonarcloud]

Quality Gate failed for 'pypowsybl'

Failed conditions
B Maintainability Rating on New Code (required ≥ A)

See analysis details on SonarCloud

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[sonarcloud]

Quality Gate passed for 'pypowsybl'

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
100.0% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarCloud