Add OmegaConfLoader

[merelcht]

Description

Added a new config loader class based on AbstractConfigLoader that uses OmegaConf to load and merge configuration.

Background 🔍

• Assess OmegaConf as replacement for anyconfig #1657
• Add a new configloader with OmegaConf as replacement for anyconf  #1868

Development notes 🏗

Some differences between the OmegaConfLoader and the other config loader classes:

• It's not recommended to use YAML anchors with OmegaConf, because they offer variable interpolation. Hence I didn't copy over the test test_ignore_hidden_keys
• YAML parsing is done a bit differently. I added ScannerError with ParserError to catch this.

Other notes:

• While writing a test for checking whether a file has the .yml or .yaml extension, I found out that OmegaConf can also handle .json files. So I've added .json as an acceptable extension for this loader as well.
• After refactoring my initial implementation, I made sure not to use any of the helper methods from config/common.py. The methods in there are deeply nested and hard to follow. Usage of these methods makes it pretty much impossible for users to customise any configloader implementations.

Review notes ✍️

• I added changes incrementally, so you can review this PR by checking out each individual commit.
• All commits starting from 1c12a87 are a refactor to not use any methods from config/common.py

To Discuss 🗣

❓Which built-in resolvers should be turned off?

We decided to initially turn off all build-in resolvers. It's easier to introduce them step by step, but removing them would be a breaking change. In #1909 we will introduce environment variables for credentials. You can find a description of all built-in resolvers here: https://omegaconf.readthedocs.io/en/2.2_branch/custom_resolvers.html#built-in-resolvers
I feel like maybe it's okay to keep some resolvers turned on. For example oc.dict seems useful and not something we wouldn't want to add. I would like to hear what others think.

❓ Is OmegaConfLoader the best name?

I first wanted to call this config loader YAMLConfigLoader but then I found out that OmegaConf can also load .json files. Any better name suggestions?

❓ Will OmegaConfLoader replace ConfigLoader?

Follow up on the above question. I'm thinking that in 0.19.0 we should remove the ConfigLoader and TemplatedConfigLoader and rename this new OmegaConfLoader to ConfigLoader. If we do that, maybe the name of this new class doesn't matter so much for the 0.18 series.

Checklist ✅

• Read the contributing guidelines
• Opened this PR as a 'Draft Pull Request' if it is work-in-progress
• Updated the documentation to reflect the code changes
• Added a description of this change in the RELEASE.md file
• Added tests to cover my changes

[noklam]

Leave 1st round of comments - haven't fully understood the code but since this is a relatively large PR, it's nice to get some early comments.

[SajidAlamQB]

This looks great thank you for this awesome work! Just had a few comments.

[SajidAlamQB]

This looks great! 🌟 I am happy for it to stay named OmegaConfLoader temporarily until 0.19.0 when we can rename it to ConfigLoader and remove the old TemplatedConfigLoader and ConfigLoader.

[noklam]

It is cleaner now with some of the methods moving into the class. I just re-iterate on one of the comment, generally look great!

[noklam]

Thank you for explaining! Nice work🌟