Remove use of simple executor in WebSocket clients

Issue: SPR-11580
Fix minor issue in STOMP broker relay
Rename handshakeAttributes to just attributes

Issue: SPR-11566
Add session attributes to SimpMessageHeaderAccessor

This change exposes the WebSocketSession attributes through a message header.
The StompSubProtocolHandler adds this to incoming messages.
For now messaging handling  methods can access the map via @Header, e.g.:

@Header(StompHeaderAccessor.SESSION_ATTRIBUTES) Map<String, Object> attrs) {

Issue: SPR-11566
Support meta-annotation overrides in ASM processing

Prior to this commit, Spring supported meta-annotation attribute
overrides in custom composed annotations with reflection-based
annotation processing but not with ASM-based annotation processing.

This commit ensures that meta-annotation attribute overrides are
supported in AnnotationMetadataReadingVisitor.getAnnotationAttributes().

Issue: SPR-11574
Upgraded embedded ASM version to 5.0

Issue: SPR-11239
Polish ComponentScanParserTests and clean up warnings