[VEF]: Support parameterized custom types

[villagesql-adam]

Support data types that don't have a fixed size, such as strings or binary data, to provide more flexibility for extension developers.

[villagestevers]

So far:
33f8f31
dbb2214

[villagestevers]

#50

[villagestevers]

#79

[villagestevers]

#104
#107

[villagestevers]

#108

[villagestevers]

#128

[villagestevers]

#131
#151
#154
#134
#136

[villagestevers]

#143

[villagestevers]

#159

[villagestevers]

This is effectively the first phase of variable length custom type support. There will be an additional series of changes to cover some type disambiguation that is needed to complete the feature.