Support interface-specific hostname on ethernet interface.

[x] This PR is for the dev branch rather than for the release branch.
[x] This PR is compliant with the other contributing guidelines as well (if not, please describe why).
[x] I have thoroughly tested my contribution.
[x] The code changes are reflected in the documentation at docs/*.

Currently only the WiFi station interface supports setting a custom hostname, overriding the system default. This PR extends the functionality to the Ethernet interface as well.

nodemcu / nodemcu-firmware

Support interface-specific hostname on ethernet interface. #3487