Read field mapping from toml

[arl]

❓ What

This is another step in the direction of making baker a full-featured binary one can use without touching code.
This gives the possibilty to defined field mapping in TOML. What we call fIeld mapping is a two-way mapping from field indexes and field names.
Before this change, one had to provide 2 function FieldName and FieldByName into baker.Components, even for super basic use of baker. Now you can specify this in TOML, from which 2 closures are generated and assigned into these 2 functions. This is done in NewConfigFromTOML.

🔨 How to test

1. List all steps necessary;
2. To test this pull request.

✅ Checklists

This section contains a list of checklists for common uses, please delete the checklists that are useless for your current use case (or add another checklist if your use case isn't covered yet).

• Is there unit/integration test coverage for all new and/or changed functionality added in this PR?
• Have the changes in this PR been functionally tested?
• Has make gofmt-write been run on the code?
• Has make govet been run on the code? Has the code been fixed accordingly to the output?
• Have the changes been added to the CHANGELOG.md file?
• Have the steps in CONTRIBUTING.md been followed to update a Go module?

[tommyblue]

resolved the if, lgtm

[tommyblue]

perfect