Missing support for only-free-headers in some relays

[svyatonik]

We're introducing new relayer mode in #2871 - it is a mode when:

• standalone GRANDPA finality relayer only relays mandatory headers and headers which numbers are divisible by some N, configured at target chain runtime;
• standalone parachains relayer, which will only relay parachain heads, available at relay chain headers, which numbers are divisible by some N.

Those submissions are free for relayer. However, we also have a plenty of other relayer modes, including: complex relayers, on-demand relayers, complex batch relayers and so on. I'm not going to add support of free-only submissions to those relays - we are only interested in standalone relayers. This issue is to mention that fact and if we'll need it some day, to add support for that.

[svyatonik]

Not actual with #3002