Scope global condition context keys and operators for #11

[dnmfarrell]

#11 says "support conditions" generically, but AWS ships ~30 global condition context keys (aws:SourceIp, aws:MultiFactorAuthPresent, aws:CurrentTime, aws:SourceVpce, aws:SecureTransport, etc.) plus multiple condition operators (StringEquals/StringLike, IpAddress, DateGreaterThan, Bool, Null, ForAllValues, ForAnyValue, the *IfExists variants).

Worth deciding scope before implementing #11's engine, since the operator set (especially ForAllValues/ForAnyValue and IfExists, which change how multi-value condition keys are evaluated) affects the design of the request-context and evaluation predicates.

Sub-tasks:

• Decide which global condition keys to support in v1 vs later.
• Decide which condition operators to support in v1 vs later.
• Feed this into the design of Support conditions #11.