Exists query

[limick]

Note: Since I no longer have access to the previous repository, this replaces PR #1070 . I've applied the suggestions from that PR and I'm updating the changelog files to reflect the new PR number.

This addresses #1069 by adding an exists function to the PersistQueryRead type class, which already provides the count function. The count function will be used for a fallback implementation for backends that don't provide a separate existence check.

Before submitting your PR, check that you've:

• Bumped the version number (can we get this into 2.11?)
• Documented new APIs with Haddock markup
• Added @since declarations to the Haddock

After submitting your PR:

• Update the Changelog.md file with a link to your PR
• Check that CI passes (or if it fails, for reasons unrelated to your change, like CI timeouts)

[parsonsmatt]

Wonderful, thanks!