add get_or_insert function to cache layer

[kaplanelad]

Adding the get_or_insert function to the cache driver for getting the option to get or insert in an easy way.

I have also added to test_cfg a dummy app context for testing

[jondot]

I assume there is no way to capture ctx right? (this is why its passed to the function)
If possible to get something like this:

.get_or_insert("key", async || {
     Users.find(..&ctx).await
})

this would be the most realistic. notice that

1. F must be async because to be realistic people would probably make async db calls there
2. ctx if possible, should be captured to avoid passing it as a parameter just to "ship it" to F
3. we might have a challenge with async closure (IIRC its unstable), so might need to see what other libs/frameworks do here

Capturing ctx would probably require some concept of sync and send, so might require wrapping ctx in Arc and/or Mutex

[jondot]

can you try a test where the async block captures something from app_ctx? like try a real DB call for example

[kaplanelad]

Added endpoint usage example + adding tests

[jondot]

something still seems off to me w/errors.

i believe, async block should just throw the regular Result from Loco (i.e. CacheError does not belong in that async block)

the block runs like any other "linear" code flow, only that in our case, the result of the block is cached. So the block can return a result or fail, like any other code flow. The failure should be for any original reason the block itself has failed.

a good way to look at it:

if you remove the cache completely, the code block should mostly stay the same

[kaplanelad]

got you, changed for supporting it