Change `ec validate input` to use positional arguments

[lcarva]

The ec validate input should take positional arguments instead of named arguments for the list of files to validate, i.e. ec validate input spam.yaml instead of ec validate input --file spam.yaml

Why? Because of all the arguments that command accepts, the input files are likely the thing to have a variable length. Consider the case where you have a bunch of yaml files in a directory. Assuming a shell is being used, positional arguments would look like ec validate input *.yaml, and named parameter like ec validate input $(find . -name '*.yaml' -printf '--file=%p ') . The second version relies on the find utility which may not be available in some contexts.

See cobra's docs on positional arguments.

Acceptance Criteria

• Change ec validate input to take positional arguments. Each is treated as a file to be validated.
• For backwards compatibility, continue to accept the --file parameter, but be sure to mark it as deprecated. If both --file and positional arguments are provided, combine the values.