pseq: Add format_utils namespace

Add a namespace containing utility functions for formatting data.

The initial utility function converts a span of container elements into
a string.

This function is a temporary solution until g++ implements the C++23
std::format() support for formatting ranges.

fmt::format() supports formatting spans, but the OpenBMC project is
converting to use of std::format() instead.

Tested:
* Ran all new and existing gtests.

Change-Id: Ie84b59db35bb49e3f1bb529490aaa05af434b6fd
Signed-off-by: Shawn McCarney <shawnmm@us.ibm.com>
diff --git a/phosphor-power-sequencer/test/meson.build b/phosphor-power-sequencer/test/meson.build
index 6bce451..9d1547b 100644
--- a/phosphor-power-sequencer/test/meson.build
+++ b/phosphor-power-sequencer/test/meson.build
@@ -2,6 +2,7 @@
      executable('phosphor-power-sequencer-tests',
                 'config_file_parser_error_tests.cpp',
                 'config_file_parser_tests.cpp',
+                'format_utils_tests.cpp',
                 'rail_tests.cpp',
                 dependencies: [
                     gmock,