Move Struct<->base::Value helpers into ChromeOsEnterpriseParams

We have a couple of locations where code needs to convert ChromeOS
enterprise params to/from a dictionary so this CL moves the code into
the struct (since it is complex and requires a .cc file now).

In addition to the re-factoring, I am also removing the code which allowed passing Enterprise params via mojom messages. This code was added to support debugging for LaCrOS, makes the impl more complicated
and no longer provides any value so I've removed it and updated the
tests.

Bug: 390189632
Change-Id: I7aa237991f36f5597b48f9beca9439dea2c5f0c7
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6226500
Commit-Queue: Joe Downing <joedow@chromium.org>
Reviewed-by: Yuwei Huang <yuweih@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1415696}