Add 'extensions' support to spec

[awoods]

Per the Editors Meeting on 2019-10-29, this task is to add language to the OCFL specification that details support for optional extensions.

The language should include:

• description of what goes in the Storage Root (including naming conventions)
• description of what goes in the Object Root (including naming conventions)
• description of scope and limitation of extensions (i.e. can not change other elements of an OCFL Object)

We may be able to repurpose some of the language currently found in the log description.

[rosy1280]

related to WIP PR: #404

[awoods]

Additional reference: https://github.com/OCFL/spec/wiki/2020.01.21-Editors-Meeting

[awoods]

Before proposing specification language, I would like to establish consensus on the details of what we are attempting to describe and define as it relates to "extensions".

1. Types of extensions
   1. Extensions referenced in specification, examples:
      • ocfl_layout.json
      • digest algorithms
   2. Extensions that require a workspace, under a subdirectory of 'extensions/', example:
      • mutable head
2. Questions to be answered
   1. How to reference extensions?
      • As sub-directory of 'extensions' directory, sharing name of extension definition
      • As a key in a specification-defined file (fixity-block, ocfl_layout.json)
   2. Where is extension referenced?
      • In 'ocfl_layout.json'
      • In 'fixity-block'
      • As sub-directory of 'extensions' directory
   3. Where does extension definition live?
      • In extensions repository
      • In the Storage Root
      • Naming convention: 4-digit, zero-padded prefix; should use hyphens to separate words; and have the .md extension.
   4. Where is extension workspace?
      • In sub-directory of 'extensions' directory of Storage Root and/or Object Root
      • Sub-directory must share the extension name, minus the '.md' suffix

References

• https://github.com/OCFL/spec/wiki/2020.01.21-Editors-Meeting#extensions
• https://github.com/OCFL/spec/wiki/2019.10.29-Editors-Meeting#notes
• https://github.com/OCFL/extensions
• https://github.com/OCFL/extensions/pull/1/files
• WIP: Extensions #404
• How to cite extensions #387
• Advice on ocfl_layout.json contents #351
• https://github.com/birkland/ocfl-rfc-demo

[zimeon]

IMO in point one we don't want to go into much detail of what type of extension there may be as that would appear to bless some directions and speak against others. I thought this ticket was specifically for description of the extensions directory and this not really about digest algorithms and such which are already described. In order to get something that is readily mergeable I would be tempted to rename 4.4 to Storage Root Extensions and add a new 3.X Object Extensions, then later think about what common language or cross-reference there should be