Reduce minor type instability issues

[mewilhel]

• [ ] Parameterize the EAGO.Optimizer with an ExtensionType and also would simplify f(m.ext_type, m) calls.
• [ ] relax_tag field can be inferred from Optimizer parameter.
• [ ] relaxed_optimizer can we infer this type from an extension?
• [ ] upper_optimizer can we infer this type from an extension?

BufferedNonlinearFunction & NonlinearExpr induce a few allocations but require a large re-write

[mewilhel]

Resolved by v0.7.