Whitelist HashWithIndifferentAccess for field serialization

[steveyken]

We currently store field configuration in the table using the YAML coder and HashWithIndifferentAccess class.

This PR ensures HashWithIndifferentAccess is whitelisted in the YAML coder.

A future PR could migrate away from HashWithIndifferentAccess and use Ruby's standard Hash class instead. This would require a migration script to ensure all Hash keys were converted and thoroughly consistent throughout the all. i.e all to string or all to symbol.

[steveyken]

Note there are potential security downsides (denial of service caused by immutable symbols causing memory exhaution) related to enabling HashWithIndifferentAccess in the YAML coders across the board. However, I believe our app entry points for YAML in general are related to the settings.yml file (controlled by administrator) and custom field administration (controlled by CRM admins). Therefore it seems like a sensible trade-off to continue supporting the existing feature until a PR to migrate away is proposed.