libobs/util: Add seeking support to array serializer

[derrod]

Description

Adds support for the seeking functionality of the OBS serializer API to the array serializer.

Also adds a reset function that can be used to avoid reallocating memory when multiple things need to be serialized in one step.

Motivation and Context

Sometimes you need to be able to seek in memory to make things easier, e.g. to write a size field after the contents have been written.

Split out from #10608 for easier review and slimming down that PR as things get merged.

How Has This Been Tested?

Tested as part of #10608.

Types of changes

• New feature (non-breaking change which adds functionality)

Checklist:

• My code has been run through clang-format.
• I have read the contributing document.
• My code is not on the master branch.
• The code has been tested.
• All commit messages are properly formatted and commits squashed where appropriate.
• I have included updates to all appropriate documentation.

[RytoEX]

Seems okay at a glance. Would appreciate another sanity check or review.