[ag] io: opt-in dependency to fmt, csl::typeinfo, csl::ensure

[GuillaumeDua]

Describe the issue

As-is, the csl::ag formatting implementation relies on:

• fmt
• csl::typeinfo
• csl::ensure

but also some gcl relicates.

What must be done ?

Comes with a better design. External dependencies in csl must be opt-in.
Auto-detect is an option for the default value though.

Dependencies

• 57