hasCapability method
Whether this implementation supports the given capability. Lets
callers detect transport limitations at runtime — for example, the
browser native EventSource does not support
SSECapability.requestHeaders, so callers that need to authenticate
must fall back to a URL-based scheme.
The default implementation returns false for every capability so
adding a new capability constant (or adding this method to an
existing external implementation) is source-compatible. Concrete
implementations should override and report what they actually
support.
Implementation
bool hasCapability(SSECapability capability) => false;