Switch integration field types from strings to symbols

Summary

The following discussion from !90704 (comment 1006225775) should be addressed:

Something minor I noticed you may want to consider with the DSL in general: the type field is likely better implemented as a symbol than a string. While freezing strings reduces memory use, symbols are just ints at the VM level so are much faster to compare. A type check in particular sounds like it will primarily be used in comparisons.

Strings on the other hand always require Ruby to walk it byte-by-byte when compared. A drawback is that symbols cannot be GC'ed since they are held in a special memory area in the Ruby VM (a hash table.) But if the set is small and immutable, this is likely worth it.

Here is a good article comparing the two approaches side by side: https://dmitrytsepelev.dev/why-has-ruby-symbols

Improvements

Convert all type: attributes from strings to symbols, and enforce this in Integrations::Field.

Maybe we could also consider decoupling the form "type" from the value type, and map them to each other:

• type: :string -> <input type="text">
• type: :text -> <textarea>
• type: :boolean -> <input type="checkbox">
• type: :secret (or :encrypted?) -> <input type="password">
• type: :select (or :array?) -> <select>

Risks

The type is used by the frontend to determine how to render the fields.