Draft of wrapping more LCP license functionality into the library instead of handlers.

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Goal: Wrap more of the functionality of LCP server into exported functions that can be used as a library from other Go services.

For example it seems useful to wrap validation things like checkGetLicenseInput, copyInputToLicense, buildLicense, etc inside a single, exported function that can be called from outside an HTTP handler.

Context:

• We'd like to use the LCP/LSD servers as libraries in our existing Go project so we don't have the additional Ops overhead of running additional servers
• I am assuming will we want input validation on partial licenses and other inputs to the LCP license handlers - this was the motivation for moving more things into the exported license.go functions

Some other changes in this PR:

• A couple of the HTTP response codes in the license handlers has changed. To try to not change them too much, I introduced a new Error that wraps more detailed input-validation error messages