Redpanda should properly decode path parameters vs query parameters

[michael-redpanda]

Version & Environment

Redpanda version: (use rpk version): all versions

What went wrong?

The + symbol is being improperly decoded to become a space character when it's found in a path parameter when path parameters are decoded using seastar's url decode method.

What should have happened instead?

Replacing a + with a space is valid for query parameters, but not path parameters. A + in the path should be left as is.

Need to create a path_decode method to be called specifically for path parameters.

Additional information

Related to #16124