Add commonly announced Capabilities to NIOIMAP.Capability

[danieleggert]

This would reduce the need to use the .other case.

Here’s a list of commonly used capabilities:

ACL
ANNOTATE-EXPERIMENT-1
AUTH=ATOKEN
AUTH=PLAIN
AUTH=PTOKEN
AUTH=WETOKEN
AUTH=WSTOKEN
BINARY
CATENATE
CHILDREN
CONDSTORE
CONTEXT=SEARCH
CONTEXT=SORT
CREATE-SPECIAL-USE
ENABLE
ESEARCH
ESORT
ID
IDLE
IMAP4rev1
LANGUAGE
LIST-STATUS
LITERAL+
LOGIN-REFERRALS
METADATA
MULTISEARCH
NAMESPACE
QRESYNC
QUOTA
RIGHTS=tekx
SASL-IR
SEARCHRES
SORT
SORT=DISPLAY
SPECIAL-USE
STATUS=SIZE
THREAD=ORDEREDSUBJECT
THREAD=REFERENCES
UIDPLUS
UNSELECT
URL-PARTIAL
URLAUTH
UTF8=ACCEPT
WITHIN
XAPPLEPUSHSERVICE
XLIST
XSNIPPET=FUZZY
XUM1

Additionally, it’d be helpful to add an enum for common sub-values, e.g.

extension NIOIMAP {
    public enum Capability: Equatable {
        case auth(Auth)
        …

        enum Auth: Equatable {
            case aToken
            case plain
            case pToken
            case weToken
            case wsToken
            case other(String)
        }
    }
}

[Davidde94]

Sounds like a good idea to me. To promote API stability we'll likely convert this to a struct but will provide convenience static functions for spec-defined capabilities.