feat(aws): apply common tags to all resources

[vdice]

• Adds a set of common_tags (in this PR currently we just have one) to be applied to all resources, enabling manual cleanup if needed.

Two considerations:

• I initially started down the default_tags path; however, with this approach, the tag values need to be known prior to apply, i.e. static. If we're okay with something static like "FermyonInstaller" = true, then we could go this route and it would clean up the code a bit (remove the merge of common_tags and resource-specific tags)
• @lann initially suggested a tag with a timestamp as a value. I opted not to go this route as it would both require updates to all applicable resources on any/every terraform apply and the tag value itself didn't seem conducive to search with... however, I'm open to pushback on this if we think tracking installation (or, update) times would be helpful.

Closes https://github.com/fermyon/installer/issues/68

[vdice]

Indeed, having a unique tag other than the instance/installation name might be handy for differentation -- but I think we'd also need to update our naming approach as well (e.g. append a unique string) if we'd like to fully eliminate the possibility of collisions between, say, two attempted installations using the same instance name. We can wait and see if demand crops up for this...