fix(encode_proto): When encoding strings into numeric proto fields, attempt to convert them automatically

[flavioc]

Summary

Before this change, attempting to convert a VRL value that contained bytes into a numeric proto field would fail since we expect the VRL value to be numeric too. This PR makes encode_proto automatically convert those strings into numeric values automatically. We still fail if the format is not valid.

Change Type

• [x] Bug fix
• [ ] New feature
• [ ] Non-functional (chore, refactoring, docs)
• [ ] Performance

Is this a breaking change?

• [ ] Yes
• [x] No

How did you test this PR?

Does this PR include user facing changes?

• [x] Yes. Please add a changelog fragment based on our guidelines.
• [ ] No. A maintainer will apply the "no-changelog" label to this PR.

Checklist

• [x] Our CONTRIBUTING.md is a good starting place.
• [x] If this PR introduces changes to LICENSE-3rdparty.csv, please run dd-rust-license-tool write and commit the changes. More details here.
• [x] For new VRL functions, please also create a sibling PR in Vector to document the new function.

References