Initial Django inspired component loading

[ixje]

What current issue(s) does this address, or what feature is it adding?
A different approach for loading plugins as suggested in #699, Django inspired

How did you solve this problem?
add a class loader

How did you make sure your solution works?
manually run api_server.py with Rest and RPC servers

Are there any special changes in the code that we should be aware of?
Yes; the RPC_SERVER and REST_SERVER variables are now stored at module level and not part of the SettingsHolder class. The implication is that all networks (main/test/priv) use the same settings. The alternative is moving it into the SettingsHolder which means we need to update all protocol.xxx.json files to include new keys to load from and otherwise have a sane default.

What approach would be preferred here (requesting extra opinions 🙏 ): @metachris @jseagrave21 @ThomasLobker @dethos
choice 1: all networks have the same plugin loading settings
choice 2: all network can have individual plugin loading settings as defined in their protocol.xxx.json

Please check the following, if applicable:

• Did you add any tests?
• Did you run make lint?
• Did you run make test?
• Are you making a PR to a feature branch or development rather than master?
• Did you add an entry to CHANGELOG.rst? (if not, please do)

[ixje]

PS: Don't merge yet. If this was the intended idea then I'll also have to update the documentation to remove any ExtendedJsonRpc references and make some note that there will be/is a plugin for it

[coveralls]

Coverage decreased (-0.05%) to 83.768% when pulling 1836a60 on ixje:plugin_django_style into ec96ae7 on CityOfZion:development.

[jseagrave21]

I may not fully understand the implications, but I think simpler is better. If we could make one change to update the mainnet, testnet, privnet settings between "default" and "extended", I think that is okay, since a user is usually only using one instance at time.

Either way, I think good documentation will allow for a user to operate the servers in the new way regardless of the implementation.

[localhuman]

Looks good! I think the only problem is that a user who installed from pip could not specify which plugin to use without editing the installed settings file.

I think choice 2 would allow for that. As long as we have sensible defaults for when plugins aren't specified in protocol.xxx.json we should be ok to allow for configuring plugins via that file.

[ixje]

This should be it, I didn't see any documentation that referenced the ExtendedJsonRpc server.

[localhuman]

Looks great!

[jseagrave21]

PS: Don't merge yet. If this was the intended idea then I'll also have to update the documentation to remove any ExtendedJsonRpc references and make some note that there will be/is a plugin for it

@ixje Are you adding instructions in this PR on how to use the new setup? Or could you implement those instructions with #664?

[ixje]

For the time being I'm not adding the instructions with this PR. We should document this at a point though.

[jseagrave21]

If you like, I can incorporate instructions into #664.