DefaultTypeAdapter: Add support for missing custom scalars

[alvaroaleman]

The default type adapter already supports some custom scalar types but not all, this change adds the missing ones. The most only used one is likely string.

Fixes https://github.com/google/cel-go/issues/884

Pull Requests Guidelines

See CONTRIBUTING.md for more details about when to create a GitHub [Pull Request][1] and when other kinds of contributions or consultation might be more desirable.

When creating a new pull request, please fork the repo and work within a development branch.

Commit Messages

• Most changes should be accompanied by tests.
• Commit messages should explain why the changes were made.

  
  Summary of change in 50 characters or less

Background on why the change is being made with additional detail on consequences of the changes elsewhere in the code or to the general functionality of the library. Multiple paragraphs may be used, but please keep lines to 72 characters or less.

## Reviews

* Perform a self-review.
* Make sure the Travis CI build passes.
* Assign a reviewer once both the above have been completed.

## Merging

* If a CEL maintaner approves the change, it may be merged by the author if
  they have write access. Otherwise, the change will be merged by a maintainer.
* Multiple commits should be squashed before merging.
* Please append the line `closes #<issue-num>: description` in the merge message,
  if applicable.

[1]:  https://help.github.com/articles/about-pull-requests

[TristonianJones]

/gcbrun

[TristonianJones]

/gcbrun

[TristonianJones]

/gcbrun