[Resolver] Basic Logic for Filtering & Sorting added

[DEKHTIARJonathan]

This PR introduces the basis for the variant resolver inside variantlib.

It attempts to define a start of User Configuration file format:

variants.toml:

# ============= Top-Most Priority ============= #

# 1. Define the variant properties `VariantProperty` to "bump to the top of the priority list"
#     Expected format: "namespace::feature::value"
# OPTIONAL: In most cases - the user will not specify the following
# Note: This is a lazy-list: Only specify the ones you want to "bump up" to the top of the list
property_priority = [
    "fictional_hw::architecture::mother",
    "fictional_tech::risk_exposure:25",
]

# ============= Second-Most Priority ============= #

# 2. Define the variant features: `VariantFeature` to "bump to the top of the priority list" 
#     These come after the ones defined in 1) [`property_priority`]
# Expected format: "namespace::feature"
# OPTIONAL: In most cases - the user will not specify the following
# Note: This is a lazy-list: Only specify the ones you want to "bump up" to the top of the list
features_priority = [
    "fictional_hw::architecture",
    "fictional_tech::risk_exposure",
    "simd_x86_64::feature3",
]

# ============= Default Priority Ordering ============= #

# 3. Define the variant namespaces by priority
# MANDATORY: as long as there is more than 1 variant plugin installed. This field must be specified
#                          If not one installed plugin is missing => should error out. 
#                          Namespaces can be specified but not installed.
namespaces_priority = [
    "fictional_hw",
    "fictional_tech",
    "simd_x86_64",
    "non_existent_provider",
]

Open Question:

• The user configuration detection relies on the "third-party library" platformdirs. Shouldn't be a problem given that pip already vendors it: https://github.com/pypa/pip/tree/main/src/pip/_vendor/platformdirs. Do we agree on that ?
• Same remark with tomllib unsupported with Python < 3.11 - Already vendored in pip under: import pip._vendor.tomli as tomllib

[DEKHTIARJonathan]

This PR is linked to the following discussion: wheelnext/pep_xxx_wheel_variants#21

[mgorny]

Hm, do I understand correctly that this currently only accounts for explicit namespace, feature, property priorities specified by user, but doesn't define fallback feature and property priorities based on the order returned by plugins?