Allow reporting the float value from external scaler

[jkremser]

Some external scalers may have report metric as a float instead of int. For example imagine a rate function.

Fixes: #5159

[zroubalik]

/run-e2e external
Update: You can check the progress here

[zroubalik]

/run-e2e external
Update: You can check the progress here

[JorTurFer]

I like this idea, but I think that this change breaks the protobuffer contract (and it's the reason behind the failing e2e).
I'm not an expert in gRPC so maybe I'm missing some important point, but I've found this page -> https://protobuf.dev/getting-started/pythontutorial/#extending-a-protobuf

Sooner or later after you release the code that uses your protocol buffer, you will undoubtedly want to “improve” the protocol buffer’s definition. If you want your new buffers to be backwards-compatible, and your old buffers to be forward-compatible – and you almost certainly do want this – then there are some rules you need to follow. In the new version of the protocol buffer:

• you must not change the tag numbers of any existing fields.
• you must not add or delete any required fields.
• you may delete optional or repeated fields.
• you may add new optional or repeated fields but you must use fresh tag numbers (that is, tag numbers that were never used in this protocol buffer, not even by deleted fields).

I think that we have to make the new proto field as optional in order to not break the contract.

Another thing that comes to my mind, as the metric value can be float, does it make sense to support floats also for targetSize?