When we do `satellite run api --placement '...'`, the placement rules are not parsed well.
The problem is based on `viper.AllSettings()`, and the main logic is sg. like this (from a new unit test):
```
r := ConfigurablePlacementRule{}
err := r.Set(p)
require.NoError(t, err)
serialized := r.String()
r2 := ConfigurablePlacementRule{}
err = r2.Set(serialized)
require.NoError(t, err)
require.Equal(t, p, r2.String())
```
All settings evaluates the placement rules in `ConfigurablePlacementRules` and stores the string representation.
The problem is that we don't have proper `String()` implementation (it prints out the structs instead of the original definition.
There are two main solutions for this problem:
1. We can fix the `String()`. When we parse a placement rule, the `String()` method should print out the original definition
2. We can switch to use pure string as configuration parameter, and parse the rules only when required.
I feel that 1 is error prone, we can do it (and in this patch I added a lot of `String()` implementations, but it's hard to be sure that our `String()` logic is inline with the parsing logic.
Therefore I decided to make the configuration value of the placements a string (or a wrapper around string).
That's the main reason why this patch seems to be big, as I updated all the usages.
But the main part is in beginning of the `placement.go` (configuration parsing is not a pflag.Value implementation any more, but a separated step).
And `filter.go`, (a few more String implementation for filters.
https://github.com/storj/storj/issues/6248
Change-Id: I47c762d3514342b76a2e85683b1c891502a0756a
98921f9faa