feat(api): external cpp header for circuits

[dbanks12]

Description

• Blocks chore(barretenberg): use a single bberg header & cleanup includes aztec-packages#720
• Closes Create single external header for use by circuits #495
• Create external API header for consumption by circuits
• Add some missing includes discovered along the way

After this PR is merged followed by its blocked aztec PR, .aztec-packages-commit can be set to master or latest aztec master commit.

Checklist:

• I have reviewed my diff in github, line by line.
• Every change is related to the PR description.
• I have linked this pull request to the issue(s) that it resolves.
• There are no unexpected formatting changes, superfluous debug logs, or commented-out code.
• There are no circuit changes, OR specifications in /markdown/specs have been updated.
• There are no circuit changes, OR a cryptographer has been assigned for review.
• I've updated any terraform that needs updating (e.g. environment variables) for deployment.
• The branch has been rebased against the head of its merge target.
• I'm happy for the PR to be merged at the reviewer's next convenience.
• New functions, classes, etc. have been documented according to the doxygen comment format. Classes and structs must have @brief describing the intended functionality.
• If existing code has been modified, such documentation has been added or updated.

[ludamad]

As noted, I never intended
#include "serialize/cbind.hpp"
#include "serialize/msgpack.hpp"
#include "serialize/test_helper.hpp"

To be exposed to header files. It may slow down compilation (although only really if we have a lot of .cpp files). Otherwise LGTM.

[codygunton]

Thanks for this. This is crucial as a first step, and in that sense it looks right to me. That said, this API is really large and hence we can expect regular breaking changes until we can pare it down as discussed earlier today.