Homogenize const types with usage in action and type storage

[tfoote]

This is a follow up to the Aggregate Foxy Message API Review

In both Marker and ImageMarker the constants are all uint8

common_interfaces/visualization_msgs/msg/ImageMarker.msg

Lines 1 to 8 in 519e851

	uint8 CIRCLE=0
	uint8 LINE_STRIP=1
	uint8 LINE_LIST=2
	uint8 POLYGON=3
	uint8 POINTS=4
	uint8 ADD=0
	uint8 REMOVE=1

However the action and type they are used in are int32

common_interfaces/visualization_msgs/msg/ImageMarker.msg

Lines 15 to 18 in 519e851

	# One of the above types, e.g. CIRCLE, LINE_STRIP, etc.
	int32 type
	# Either ADD or REMOVE.
	int32 action

This proposes to homogenize them by changing the const's declared type to be consistent.

[dirk-thomas]

Was the alternative considered - to change the type of the type and action fields. 8 bit seems more than enough to encode the possible values. If both options were considered I think it would be good to mention the rationale here why one was chosen over the other.

[tfoote]

That would be possible but would be changing the message format. Which would then require bag migrations and other mechanisms. Right now the consts are just being implicitly cast to the larger type when assigned into the message and consequently this is the least disruptive resolution.

[tfoote]

Rebased with basic CI to test code changes

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