Move `headersSplitValues` into `BaseClient`

[brianquinlan]

Move headersSplitValues into BaseClient and allow the headers parameter in the BaseClient constructor to be Map<String, String> or Map<String, List<String>>.

---

• [X] I’ve reviewed the contributor guide and applied the relevant portions to this PR.

Contribution guidelines:

• See our contributor guide for general expectations for PRs.
• Larger or significant changes should be discussed in an issue before creating a PR.
• Contributions to our repos should follow the Dart style guide and use dart format.
• Most changes should add an entry to the changelog and may need to rev the pubspec package version.
• Changes to packages require corresponding tests.

Note that many Dart repos have a weekly cadence for reviewing PRs - please allow for some latency before initial review feedback.