Adjust MARC authority record definition to Avram format

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Following #363 and https://github.com/timathom/marc-schema/issues/1 to make class Marc21SchemaReader an Avram schema reader (at least for Marc schemas, but may be extended for PICA schemas as well).

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Missing from #371:

• rename key leader to LDR
• change object indicators with keys 1 and 2 to indicator1 and indicator2: jq '.fields[]|=(.indicators as $i|if $i then (.+{indicator1:($i|.["1"]),indicator2:($i|.["2"])}|del(.indicators)) else . end)' src/main/resources/marc/authority-schema.json
• remove key static, its value is implied by existence of codes
• change value of positions to an Avram position object

The additional value key in codes will be inclued as code in Avram specification, so this should also be renamed.