create set of link types for gmd:procotol

[tomkralidis]

Summary and Purpose
WCMP metadata provides guidance on articulating links given gmd:distribution//gmd:onLine. The gmd:protocol of gmd:CI_OnlineResource allows the user to specify the nature of the link type. For example, links to APIs benefit here, the client would have a better idea of a gmd:URL's behaviour without having to do too much content negotiation/sniffing apriori.

There have been numerous approaches across various information communities over the years regarding "link types".

Examples:

• in the OGC API - Records specification development we are working on clarifying exactly this by possibly allowing link types to be qualified by a given authority/information community (OSGeo, etc.) or directly from in the OGC Naming Authority/Registry itself.

          <gmd:onLine>
            <gmd:CI_OnlineResource>
              <gmd:linkage>
                <gmd:URL>https://geo.woudc.org/ows</gmd:URL>
              </gmd:linkage>
              <!-- OGC:WMS as per https://github.com/OSGeo/Cat-Interop/blob/master/LinkPropertyLookupTable.csv -->
              <gmd:protocol>
                <gco:CharacterString>OGC:WMS</gco:CharacterString>
              </gmd:protocol>
              <gmd:name xsi:type="gmd:PT_FreeText_PropertyType">
                <gco:CharacterString>totalozone</gco:CharacterString>
                <gmd:PT_FreeText>
                  <gmd:textGroup>
                    <gmd:LocalisedCharacterString locale="#locale-fr">totalozone</gmd:LocalisedCharacterString>
                  </gmd:textGroup>
                </gmd:PT_FreeText>
              </gmd:name>
              <gmd:description>
                <gco:CharacterString>OGC Web Map Service (WMS)</gco:CharacterString>
              </gmd:description>
              <gmd:function>
                <gmd:CI_OnLineFunctionCode codeList="http://wis.wmo.int/2011/schemata/iso19139_2007/schema/resources/Codelist/gmxCodelists.xml#CI_OnLineFunctionCode" codeListValue="download" codeSpace="ISOTC211/19115">download</gmd:CI_OnLineFunctionCode>
              </gmd:function>
            </gmd:CI_OnlineResource>
          </gmd:onLine>

Proposal
Create a minimal set of link types for WCMP purposes.

Reason
Better qualify gmd:protocol in order to provide improved interoperability for actionable links in WCMP metadata.

[tomkralidis]

cc @wmo-im/tt-wismd please provide thoughts and feedback -- thanks.