Cache `search_fields()` results within a session

[matildastevenson]

Could use memoise to do this, or could cache in the same way as downloaded data, but this would preserve results between sessions.

[mjwestgate]

Follow-up: This is important because when galah_config(run_checks = TRUE) is set, every call to select_filters or select_columns (and a few others, like ala_counts) has to run search_fields first. This is very slow for something that would otherwise simply return some parsed text.

Proposed solution is:

• add valid_fields slot to galah_config which is a length-zero vector by default (i.e. c())
• consolidate validate_fields() and validate_cols into a single function (e.g. validate_fields())
• on package load, leave galah_config(valid_fields) as empty
• on first call to validate_fields, update valid_fields
• prevent galah_config from saving/loading information from valid_fields to profile (i.e. no long-term storage)

This should work within a session without slowing package loading, while also preventing long-term storage