Assess `uw config compose` to generalize YAML serialization process

[singhd789]

Is your feature request related to a problem? Please describe.
At the moment, the fre yamltools combine-yamls subtool works well but is more specific to certain use cases. For example, it can currently combine YAMLs for --use compile, --use pp, and --use cmor. In order to add another use case, the code must be added. Initially, this seemed to be more organized and concise, but as our workflows and YAMLs grow, we desire more flexibility here.

Describe the solution you'd like

• create subtool to list desired yamls to combine
• Assess the viability of using uw config compose to replace the fre yamltools combine-yaml subtool (to actually combine desired yamls)

Note: We still want to work on per yaml validation as well.

[singhd789]

Notes:

• !join constructor doesn't seem to exist in uw tools
• if multiple pp yamls are listed for one experiment, the [postprocess][components] section is overwritten by the second pp yaml listed