General functionality for handling validation in the app

(please note that the below is just a rough design - feel free to adapt if needed)

* `module_validate_xyz()` - a shiny module executing validation:
* `srv_validate_xyz(x)` - it does the following:
```
check_1 <- module_check_foo(x, ...)
check_2 <- module_check_bar(x, ...)
check_3 <- module_check_baz(x, ...)
...
checks <- list(check_1, check_2, check_3, ...)
r_validate <- reactive({
for (check in checks) {
validate(need(check()$isEmpty(), check$getMessages()))
}
x()
})
output$errors <- renderUI({
r_validate()
NULL
})
x
```
* `ui_validate_xyz()` - display error message:
```
tagList(
uiOutput(NS(id, "errors"))
)
```
* we might assign a dedicated CSS class and apply custom styling of that element.
* The goal is to allow nesting calls of validate module from another validate module
* `module_check_xyz()` - a shiny module executing checks:
* `srv_module_check_xyz()` - returns reactive with the `AssertCollection` object (see: https://mllg.github.io/checkmate/reference/AssertCollection.html)
* `ui_module_check_xyz()` - empty fun (we can even don't create it at all)
* `check_xyz()` - for custom checks - only if needed, function returning TRUE if ok else error message (similar to `checkmate`)
* `test_xyz()` - for custom checks - only if needed, function returning boolean (similar to `checkmate`)

Extra:
I do expect quite a few `module_check_xyz()` functions, e.g. `module_check_reactive()`, `module_check_tryerror()`, `module_check_shinysilenterror()`, `module_check_qenverror`. Instead of exporting a lot of objects, we might create a factory to create them dynamically if needed. (or both?)

The goals it to have a functionality that is:
- testable
- extendable - e.g. one validate module calling another validate module
- returning consistent validation errors

The use cases:
- `teal:::srv_validate_reactive_teal_data`
- teal modules (I assume this is going to be exported)