[rpk] Add deprecation warnings for weak/unsupported types in redpanda configuration file.

[r-vasquez]

Who is this for and what problem do they have today?

We want to transition to having a configuration file with strong type check when using rpk, we have been working toward that goal and we need to start printing deprecation warnings so the end-users know where are the problems.

What are the success criteria?

Each time a user runs an rpk command that uses a config file with weak types, rpk should print a deprecation warning like:

redpanda.yaml: Server TLS must be an array; a non-array value in array fields is deprecated and will be removed in the future

Additional notes

Relevant PR: #5061

[twmb]

After sitting on this for a really long time -- I think it's ok to keep our weak decoding code and not force a deprecation. Closing.