Allow cz bump to use PEP 621 pyproject.toml/project.version

[dmyersturnbull]

Description

Currently, using PEP 621, the version needs to be stored in pyproject.toml in two locations:

• project.version
• tool.commitizen.version

Running cz bump modifies only tool.commitizen.version, leaving the two out of sync. project.version is arguably the correct place to store this metadata for anything outside of commitizen's purview.

Possible Solution

A config item / CLI option could be added to specify that commitizen should only operate on project.version.
Example:

[project]
name = "my-project"
version = "0.1.0"

[tool.commitizen]
version_key = "project.version"

### Additional context

_No response_

### Additional context

_No response_

[woile]

Already supported: https://commitizen-tools.github.io/commitizen/config/#version-providers