Add custom types for encoded strings and unit tests

[harshadixit12]

We are introducing two custom encoded string types - base64 encoded, and optionally base64 encoded.
These expect the value in plan, state to be encoded.

Fixes Kong/terraform-provider-konnect#85
Short document explaining the solution: https://konghq.atlassian.net/wiki/spaces/A/pages/4604264449/Terraform+custom+types

Corresponding PR on platform API: https://github.com/Kong/platform-api/pull/1254

How I have tested this:

• Run unit tests

[harshadixit12]

@mheap I've tested this by copying the customtypes dir to the konnect terraform provider repo.
Not able to test this by importing shared-speakeasy yet though (context)
I'd like to merge this and release before updating the PRs on platform-api and terraform-provider-konnect