Add new traffic sign variability types

[thomassedlmayer]

Description

• Clarify definition of VARIABILITY_FIXED and VARIABILITY_VARIABLE to differentiate from new types
• Add new types VARIABILITY_VARIABLE_MOVABLE and VARIABILITY_VARIABLE_MUTABLE

As already mentioned by @jdsika in #623, the differentiation between mutable content (e.g. traffic sign type) and changes in position of traffic signs is not given yet.
To maintain backwards compatibility the existing type VARIABILITY_VARIABLE is essentially kept as it is. It still describes traffic signs that CAN BE semantically and positionally variable. This means that it has to be expected that the traffic sign's type and position could change over time (exactly as it is with the current OSI version). If further differentiation in terms of semantics and position is desired, the new types VARIABILITY_VARIABLE_MOVABLE and VARIABILITY_VARIABLE_MUTABLE can be used.

Examples

VARIABILITY_VARIABLE:

Pickup truck at the Autobahn carries a semantically mutable (e.g. digital) sign for a construction site which moves on the car.

VARIABILITY_VARIABLE_MUTABLE:

Digital or analog mutable traffic signs like signs on a traffic sign gantry.

VARIABILITY_VARIABLE_MOVABLE:

Temporary (but semantically immutable) signs at construction sites.

Checklist

• My suggestion follows the style and contributors guidelines.
• I have taken care about the message documentation and the fields and enums documentation.
• I have done the DCO signoff.
• My changes generate no errors when passing CI tests.
• I have successfully implemented and tested my fix/feature locally.
• Appropriate reviewer(s) are assigned.

#### References
resolves #623
#671

[pmai]

CCB 2022-11-07: First to be further discussed in Harmonisation group, and possibly await results from dynamic signs harmonization effort.

[thempen]

The enum VARIABILITY_VARIABLE is not clearly defined, if it is movable and mutable, it can be, or can't. An additional field "VARIABILITY_MOVABLE_AND_MUTABLE" will be more specific.

Movable assumes, you can have a temporary sign e.g., for road works, but also traffic signs on vehicles. @SensorWG: Do we need to distinguish between these tempory signs or signs on vehicles?

[pmai]

Proposed changes as per harmonization discussion.

[pmai]

CCB 2022-12-12: Merge as-is.