xml: T6650: add initial op-mode cache support

[jestabro]

Change Summary

Add initial support for op-mode XML cache, analogous to the config-mode/reference tree cache.

Unlike the case of config-mode, this is not critical to basic functions of op-mode, though future developments of the op-mode daemon or other development may have a use. It is initially provided to provide companion support for online documentation, and as such, additional development of the access functions in op_definition.py will be based on the needs of that project.

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Code style update (formatting, renaming)
• Refactoring (no functional changes)
• Migration from an old Vyatta component to vyos-1x, please link to related PR inside obsoleted component
• Other (please describe):

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Checklist:

• I have read the CONTRIBUTING document
• I have linked this PR to one or more Phabricator Task(s)
• I have run the components SMOKETESTS if applicable
• My commit headlines contain a valid Task id
• My change requires a change to the documentation
• I have updated the documentation accordingly

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