Export Paramgen

[lovromazgon]

Description

This PR exports the internals from paramgen so they can be reused in specgen (#TODO).

Apart from exporting it, there were a few additions (as always):

• Support was added for external types (3c2d133), meaning that a config can now import a struct from a different package. This is needed, as we want connector configs to include the middleware config coming from the connector SDK.
• Added support for pointers in config structs. This is needed, because some middleware configs contain pointers for when the default is not the zero value and we want to let the user overwrite that default with the actual zero value.
• Fields tagged with json:"-" are now ignored by the generator. This enables adding additional exported utility fields, without affecting the generated specification.

Apart from these additions to paramgen, I added 2 more utility functions:

• lang.ValOrZero a generic utility for dereferencing a pointer and getting the zero value if the pointer is nil.
• lang.Zero a generic utility for getting the zero value for a type.

Related to ConduitIO/conduit#1523.

Quick checks:

• I have followed the Code Guidelines.
• There is no other pull request for the same update/change.
• I have written unit tests.
• I have made sure that the PR is of reasonable size and can be easily reviewed.

[raulb]

LGTM.