[Examples] add AWS credentials environment validator

[sadiqk2]

Q	A
Bug fix?	no
New feature?	no
Docs?	no
Issues
License	MIT

• Created EnvValidator to centralize AWS credentials environment variable validation
• Updated examples/bedrock/ to use the new validator

[sadiqk2]

@chr-hertel Can you review this ?

[OskarStark]

We usually haven env() shortcut in other examples, I would prefer to use this

[sadiqk2]

We usually haven env() shortcut in other examples, I would prefer to use this

@OskarStark you mean to env() instead of global var thingy

[chr-hertel]

The difference with these examples is that env(...) is usually used to access the $_SERVER and throw an error if not set. In our Bedrock bridge we don't have to do that since the aws lib we're using is fetching the env vars itself.

Again, like with #313, I'm not convinced to have an extra class here. I see the point that this looks a bit odd right now, but unless we can come up with a better idea than an extra class, i'd rather keep it like that.