Get custom fields

[NSeydoux]

Refines getters to read custom fields.

This adds throwing on deserialization issues or unexpected number of values as well.

Supporting arrays is out of scope for this.

Checklist

• All acceptance criteria are met.
• Relevant documentation, if any, has been written/updated.
• The changelog has been updated, if applicable.
• New functions/types have been exported in index.ts, if applicable.
• New modules (i.e. new .ts files) are listed in the exports field in package.json, if applicable.
• New modules (i.e. new .ts files) are listed in the typedocOptions.entryPoints field in tsconfig.json, if applicable.
• Commits in this PR are minimal and have descriptive commit messages.

[edwardsph]

LGTM