Implement ValueFormat for DAP

Implement ValueFormat for DAP

This patch implements ValueFormat for DAP.  Currently this only means
supporting "hex".

Note that StackFrameFormat is defined to have many more options, but
none are currently recognized.  It isn't entirely clear how these
should be handled.  I'll file a new gdb bug for this, and perhaps an
upstream DAP bug as well.

New in v2:
- I realized that the "hover" context was broken, and furthermore
  that we only had tests for "hover" failing, not for it succeeding.
  This version fixes the oversight and adds a test.

Bug: https://sourceware.org/bugzilla/show_bug.cgi?id=30469