stdlib, tests, configs: Introduce gem5 Vision to resources

This patch makes changes to the stdlib based on the gem5 Vision project.
Firstly, a MongoDB database is supported.
A JSON database's support is continued.
The JSON can either be a local path or a raw GitHub link.

The data for these databases is stored in src/python
under "gem5-config.json".
This will be used by default.
However, the configuration can be overridden:
- by providing a path using the GEM5_CONFIG env variable.
- by placing a gem5-config.json file in the current working directory.

An AbstractClient is an abstract class that implements
searching and sorting relevant to the databases.

Clients is an optional list that can be passed
while defining any Resource class and obtain_resource.
These databases can be defined in the config JSON.

Resources now have versions. This allows for a
single version, e.g., 'x86-ubuntu-boot', to have
multiple versions. As such, the key of a resource is
its ID and Version (e.g., 'x86-ubuntu-boot/v2.1.0').
Different versions of a resource might be compatible
with different versions of gem5.

By default, it picks the latest version compatible with the gem5 Version
of the user.

A gem5 resource schema now has additional fields.
These are:
- source_url: Stores URL of GitHub Source of the resource.
- license: License information of the resource.
- tags: Words to identify a resource better, like hello for hello-world
- example_usage: How to use the resource in a simulation.
- gem5_versions: List of gem5 versions that resource is compatible with.
- resource_version: The version of the resource itself.
- size: The download size of the resource, if it exists.
- code_examples: List of objects.
These objects contain the path to where a resource is
used in gem5 example config scripts,
and if the resource itself is used in tests or not.
- category: Category of the resource, as defined by classes in
src/python/gem5/resources/resource.py.

Some fields have been renamed:
- "name" is changed to "id"
- "documentation" is changed to "description"

Besides these, the schema also supports resource specialization.
It adds fields relevant to a specific resource as specified in
src/python/gem5/resources/resource.py
These changes have been made to better present
information on the new gem5 Resources website.

But, they do not affect the way resources are used by a gem5 user.
This patch is also backwards compatible.
Existing code doesn't break with this new infrastructure.

Also, refs in the tests have been changed to match this new schema.
Tests have been changed to work with the two clients.

Change-Id: Ia9bf47f7900763827fd5e873bcd663cc3ecdba40
Co-authored-by: Kunal Pai <kunpai@ucdavis.edu>
Co-authored-by: Parth Shah <helloparthshah@gmail.com>
Co-authored-by: Harshil Patel <harshilp2107@gmail.com>
Co-authored-by: aarsli <arsli@ucdavis.edu>
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/71278
Maintainer: Bobby Bruce <bbruce@ucdavis.edu>
Tested-by: kokoro <noreply+kokoro@google.com>
Reviewed-by: Bobby Bruce <bbruce@ucdavis.edu>
26 files changed