http api: T3412: use FastAPI as web framework; support application/json

[jestabro]

Change Summary

Replace the Flask micro-framework with FastAPI, in order to support
extensions to the API and OpenAPI 3.* generation. This change will
remain backwards compatible with previous versions. Notably, the
multipart forms version of requests remain supported; in addition
application/json requests are now natively supported.

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):

Related Task(s)

• https://phabricator.vyos.net/T3412
• https://phabricator.vyos.net/T2397

Component(s) name

http-api

Proposed changes

How to test

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

[dmbaturin]

It's a big change so I might have missed something, but so far it looks good!

[c-po]

I'm not an API expert, but this looks reasonable structured to me, we can definately give it a spin in 1.4