create and publish xml schema for the specific swh-deposit metadata

In the metadata-only spec a TODO reside with the following action: publish schema at https://www.softwareheritage.org/schema/2018/deposit

which is followed with this example:

<?xml version="1.0"?>
  <entry xmlns="http://www.w3.org/2005/Atom"
           xmlns:codemeta="https://doi.org/10.5063/SCHEMA/CODEMETA-2.0"
           xmlns:swh="https://www.softwareheritage.org/schema/2018/deposit">
      <author>
        <name>HAL</name>
        <email>hal@ccsd.cnrs.fr</email>
      </author>
      <client>hal</client>
      <external_identifier>hal-01243573</external_identifier>
      <codemeta:name>The assignment problem</codemeta:name>
      <codemeta:url>https://hal.archives-ouvertes.fr/hal-01243573</codemeta:url>
      <codemeta:identifier>other identifier, DOI, ARK</codemeta:identifier>
      <codemeta:applicationCategory>Domain</codemeta:applicationCategory>
      <codemeta:description>description</codemeta:description>
      <codemeta:author>
          <codemeta:name> author1 </codemeta:name>
          <codemeta:affiliation> Inria </codemeta:affiliation>
          <codemeta:affiliation> UPMC </codemeta:affiliation>
      </codemeta:author>
      <codemeta:author>
          <codemeta:name> author2 </codemeta:name>
          <codemeta:affiliation> Inria </codemeta:affiliation>
          <codemeta:affiliation> UPMC </codemeta:affiliation>
      </codemeta:author>
      <swh:deposit>
        <swh:reference>
          <swh:origin url='https://github.com/user/repo'/>
        </swh:reference>
      </swh:deposit>
  </entry>

We should choose a better url for the schema. I think there is a draft for that somewhere, but not sure where.

Note to change example in docs/specs/spec-meta-deposit.rst.

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Migrated from T2625 (view on Phabricator)