refactor: move plugin/fieldpath to api/types/

[akhilerm]

move the plugin/fieldpath to api/types/, so that api directory is independent of rest of the containerd module.

Manually tested if its breaking compatibility with older client by :

• building and running containerd from the PR branch which uses modified protos
• running the sample code for redis-container from here , which uses the module from main branch that uses old protos.

[k8s-ci-robot]

Skipping CI for Draft Pull Request.
If you want CI signal for your change, please convert it to an actual PR.
You can still manually trigger a test run with /test all

[akhilerm]

/test all

[akhilerm]

/cc @dmcgowan

[akhilerm]

Manually tested if its breaking compatibility with older client by :

• building and running containerd from the PR branch which uses modified protos
• running the sample code for redis-container from here , which uses the module from main branch that uses old protos.