Simplify overridable config code (#12719)

This commit simplifies the process of adding overridable configurations
by using a new OverridableConfigDefs.h file as the largely single source
of truth.

OverridableConfigDefs.h contains an X-macro that helps autogenerate the
following which used to need to be manually updated:

- Lua plugin enum and var array (ts_lua_http_config.cc)
- String-to-enum mapping (overridable_txn_vars.cc)
- _conf_to_memberp switch statement (InkAPI.cc)
- SDK_Overridable_Configs test array (InkAPITest.cc)

Files that still require manual updates when adding a new config:

- include/ts/apidefs.h.in: enum value for ABI stability.
- include/proxy/http/OverridableConfigDefs.h: X-macro entry in enum order.
- include/proxy/http/HttpConfig.h: struct member.
- docs: Documentation files.

Author: bneradt