feat: Utilize globally installed gtest if available

[phated]

Description

This is similar to #134 in that it first checks to see if GTest is available in the CMake path before it attempts to download it. I've also made sure to disable gMock, which doesn't seem to be used, if gtest is built from source.

The aliases are created to ensure the namespace stays consistent between globally installed and source builds.

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.