AP_NavEKF: add option to align ExtNav position when using OptFlow

[rmackay9]

This increases "alignment" of External Nav (aka VisualOdometry) in a couple of situations.

• EK3_SRC_OPTIONS gets a new bit which enables aligning the External Nav position to the Optical Flow based position estimate. This is useful for situations where only optical flow is being used as a backup for the external nav system and we want to avoid jumps when switching from optical flow to external nav. The downside is that any drift that has occurred while using optical flow will essentially be "locked-in". See issue Pose jump after source switching OPT_FLOW->EXT_NAV(Tertiary->Secondary) after EXT_NAV reset #23973
• External Nav's yaw is aligned at the same time as its position and velocity are aligned. This means most users will not need to use the VisoAlign auxiliary switch any more. It is hard to imagine why users would not want to align the yaw which is why I have not hidden this new feature behind an option bit.

This has not yet been tested.