Allow adding or updating etc. for WMS Attribution

[sirviot]

There are metadata fields available for lauers that may be necessary in some use cases. For example when describing licensing terms for a data. The WMS Attribution settings could be used for storing such information. It may easier to store and retrieve this kind of information from separate layer attributes instead of overloading other availble text fields.

From GeoServer documentation - WMS Attribution:

• Attribution Text—Human-readable text describing the data provider. This might be used as the text for a hyperlink to the data provider’s website.
• Attribution Link—URL to the data provider’s website.
• Logo URL—URL to an image that serves as a logo for the data provider.
• Logo Content Type, Width, and Height—These fields provide information about the logo image that clients may use to assist with layout.