[5.x] Allow for large field configs in filters

[duncanmcclean]

This pull request fixes an issue with filters, where a config field (usually a Select field with lots of options) could cause a request to go over the server's max headers size.

This can be easily resolved by sending the encryped field config in a POST request, rather than a GET request, since POST requests can handle larger payloads.

I've kept the GET version of the field-meta endpoint for now, as it's being used in third-party addons (example).

For easier testing, here's a filter with lots of select options: https://gist.github.com/duncanmcclean/171e55a861273d111c6ca037d8650aad