Deprecate Message::objectParams() and related code

This functionality was introduced in 2021 (commit 349819dc5a)
to support the addition of UserGroupMembershipParam, which was
never used, and no other use case appeared.

Its existence is now preventing us from allowing serializing
of MessageValue objects as JSON (since the parameters can't be
guaranteed to be serializable).

Deprecate:

• method: MessageValue::objectParams()
• method: Message::objectParams()
• method: Message::objectParam()
• class: UserGroupMembershipParam
• constant: ParamType::OBJECT
• Passing Stringable objects to ScalarParam

Change-Id: I492edabb7ea1d75774b45eb9fd18261b39963f9f