feat(manager/pep621): implement manager (#22082)

lib/modules/datasource/pypi/index.ts

lib/modules/manager/api.ts

...es/manager/pep621/__fixtures__/pyproject_pdm_sources.toml

...dules/manager/pep621/__fixtures__/pyproject_with_pdm.toml

lib/modules/manager/pep621/extract.spec.ts

lib/modules/manager/pep621/extract.ts

lib/modules/manager/pep621/index.ts

lib/modules/manager/pep621/processors/index.ts

lib/modules/manager/pep621/processors/pdm.ts

lib/modules/manager/pep621/processors/types.ts

lib/modules/manager/pep621/readme.md

lib/modules/manager/pep621/schema.ts

lib/modules/manager/pep621/types.ts

lib/modules/manager/pep621/utils.spec.ts

lib/modules/manager/pep621/utils.ts