DEBUGINFOD based DWP acquisition for LLDB (#70996)

I've plumbed the LLVM DebugInfoD client into LLDB, and added automatic
downloading of DWP files to the SymbolFileDWARF.cpp plugin. If you have
DEBUGINFOD_URLS set to a space delimited set of web servers, LLDB will
try to use them as a last resort when searching for DWP files. If you do
*not* have that environment variable set, nothing should be changed.
There's also a setting, per @clayborg 's suggestion, that will override
the environment variable, or can be used instead of the environment
variable. The setting is why I also needed to add an API to the
llvm-debuginfod library

### Test Plan:

Suggestions are welcome here. I should probably have some positive and
negative tests, but I wanted to get the diff up for people who have a
clue what they're doing to rip it to pieces before spending too much
time validating the initial implementation.

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Co-authored-by: Kevin Frei <freik@meta.com>
Co-authored-by: Alex Langford <nirvashtzero@gmail.com>