Rename `ExportGenesisStateCommand` to `ExportGenesisHeadCommand` and make it respect custom genesis block builders (#2331)

Closes #2326.

This PR both fixes a logic bug and replaces an incorrect name.

## Bug Fix: Respecting custom genesis builder

Prior to this PR the standard logic for creating a genesis block was
repeated inside of cumulus. This PR removes that duplicated logic, and
calls into the proper `BuildGenesisBlock` implementation.

One consequence is that if the genesis block has already been
initialized, it will not be re-created, but rather read from the
database like it is for other node invocations. So you need to watch out
for old unpurged data during the development process. Offchain tools may
need to be updated accordingly. I've already filed
https://github.com/paritytech/zombienet/issues/1519



## Rename: It doesn't export state. It exports head data.

The name export-genesis-state was always wrong, nad it's never too late
to right a wrong. I've changed the name of the struct to
`ExportGenesisHeadCommand`.

There is still the question of what to do with individual nodes' public
CLIs. I have updated the parachain template to a reasonable default that
preserves compatibility with tools that will expect
`export-genesis-state` to still work. And I've chosen not to modify the
public CLIs of any other nodes in the repo. I'll leave it up to their
individual owners/maintains to decide whether that is appropriate.

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Co-authored-by: Joshy Orndorff <[email protected]>
Co-authored-by: Bastian Köcher <[email protected]>
Co-authored-by: Bastian Köcher <[email protected]>