Add optional esmvalcore dependency & `to_esmvalcore` method

[charles-turner-1]

Changes

• [x] Add optional imports following the way polars does it to keep overhead to a minimum.
• [x] Add esmvalcore to list of optional imports.
• [x] Add to_esmvalcore method to esm_datastore class
• [x] Keep track of all searches made on a datastore
• [ ] Tests for to_esmvalcore method (will require some heavy mocking)

Related issue number

https://github.com/ESMValGroup/ESMValCore/pull/2690

Closes #715

Checklist

• [ ] Unit tests for the changes exist
• [ ] Tests pass on CI
• [ ] Documentation reflects the changes where applicable