GEP: TLS Configuration Placement and Terminology

[robscott]

What would you like to be added:
A memorandum GEP that provides a high level vision for TLS configuration in Gateway API. This GEP will not include any specific API changes, but will propose naming patterns for top level fields/buckets that should be used to TLS configuration.

Why this is needed:
We've got two TLS GEPs in flight (#91 and #1897), and likely more on the way. If we're not careful, each will end up with distinct naming and structure that made sense in isolation but does not fit the broader picture.