Add layer properties to layer type

[lucas-angermann]

PR Checklist

Please check if your PR fulfills the following requirements:

• The commit message follows our guidelines: https://github.com/dlr-eoc/ukis-frontend-libraries/blob/main/CONTRIBUTING.md#-commit-message-guidelines
• Tests for the changes have been added (for bug fixes / features)
• Docs have been added / updated (for bug fixes / features)
• The Project is building and passes all tests without errors

PR Type

What kind of change does this PR introduce?

• Bugfix
• Feature
• Code style update (formatting, local variables)
• Refactoring (no functional changes, no api changes)
• Build related changes
• CI related changes
• Documentation content changes
• Other... Please describe:

What is the current behavior?

Layers do not have an attribute for additional properties like unit.

Issue Number: 264

What is the new behavior?

New properties attribute for Layers.

Does this PR introduce a breaking change?

• Yes
• No

Other information

Is it part of one/more packages and which?
@dlr-eoc/services-layers

[boeckMt]

Thanks for the changes and the new feature :)
CLOSES: #264